Typesetter 5.1, php 5.5, Any browser. Installation goes fine, but after completing install, all the default pages are bank.  Log into index.php/Admin works, but trying to edit any of the default pages returns a blank browser screen. If I create a new layout from any of the Bootswatch Flatly themes, and apply it to a page, the page displays correctly. The same applies to creating a layout from the Three Point 5 theme. But any layout created from any Bootswatch Scss theme results in a blank page. Creating a layout from the new Cajón Scss theme in v 5.1 also results in a blank page. I've tried installing Typesetter versions 5.0.3 and 5.1 on several sites, always with the same results. I've also tried php versions 5.3, 5.4 as well as 5.5, and browsers including  Firefox, Chromium, & Opera. Same result.

If I preview a Bootswatch Scss or Cajón Scss theme or edit a Bootswatch Scss or Cajón Scss layout, the iframe in which the sample page is supposed to be displayed is blank in browser View Frame Source. When previewing/editing Bootswatch Flatly and Three Point 5 themes/layouts, browser View Frame Source shows the HTML code for the page as expected. 

Has anyone experienced this odd behaviour?

There have been blank page issues now and then - with different causes. But none of them was theme-dependent as far as I can remember.

So my first assumption would be that it has something to do with the css preprocessor/compiler and memory consumption.
Bootswatch Flatly uses LESS while both Bootswatch Scss and Cajón are Scss-based.
But, from my experience, the Scss compiler needs less memory, therefore I doubt it's a memory issue.

Do you have access to the error logs?
If you open the network tab of your web browser's develpoer tools, which response status does the blank page have. Presumably 500(?)
And another question - do you know which OS/dsitribution the server is running?

According to phpinfo, host is x86_64-redhat-linux-gnu.

Error log does not show anything, but Transfer Log does provide the following info:

When About page is accessed when not logged into admin panel (edited for privacy):

[23/Jan/2018:15:53:07 -0600] "GET / HTTP/1.1" 200 2898 "-" "Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/63.0.3239.108 Safari/537.36 OPR/50.0.2762.45"

[23/Jan/2018:15:53:07 -0600] "GET /data/_cache/combined_5n8m63p2ziu62ulxhw9tw6.js HTTP/1.1" 200 133094 "http://domain.com/" "Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/63.0.3239.108 Safari/537.36 OPR/50.0.2762.45"

[23/Jan/2018:15:53:13 -0600] "GET /index.php/About HTTP/1.1" 500 - "http://domain.com/" "Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/63.0.3239.108 Safari/537.36 OPR/50.0.2762.45"

When logged in, editing a layout and selecting Bootswatch Flatly:

[23/Jan/2018:16:16:20 -0600] "GET /index.php/Admin_Theme_Content/Edit/900masg HTTP/1.1" 200 2136 "http://domain.com/index.php/Admin_Theme_Content/Edit/default?redir=Home" "Mozilla/5.0 (X11; Ubuntu; Linux x86_64; rv:57.0) Gecko/20100101 Firefox/57.0"

[23/Jan/2018:16:16:20 -0600] "GET /index.php/Admin_Theme_Content/Edit/900masg?cmd=in_iframe HTTP/1.1" 200 4721 "http://domain.com/index.php/Admin_Theme_Content/Edit/900masg" "Mozilla/5.0 (X11; Ubuntu; Linux x86_64; rv:57.0) Gecko/20100101 Firefox/57.0"

But when selecting a Bootswatch_Scss layout:

[23/Jan/2018:16:22:08 -0600] "GET /index.php/Admin_Theme_Content/Edit/default HTTP/1.1" 200 8453 "http://domain.com/index.php/Admin_Theme_Content/Edit/900masg" "Mozilla/5.0 (X11; Ubuntu; Linux x86_64; rv:57.0) Gecko/20100101 Firefox/57.0"

[23/Jan/2018:16:22:08 -0600] "GET /index.php/Admin_Theme_Content/Edit/default?cmd=in_iframe HTTP/1.1" 500 - "http://domain.com/index.php/Admin_Theme_Content/Edit/default" "Mozilla/5.0 (X11; Ubuntu; Linux x86_64; rv:57.0) Gecko/20100101 Firefox/57.0"

The network tab for Bootswatch Flatly:

and for Bootswatch_Scss:

It might be a memory consumption issue. I experimented with include/thirdparty/Bootstrap3/scss/_bootstrap.scss commenting out various combinations of @import. For example commenting out all Components w/ JavaScript allows the page to load correctly. I've checked that it's not these components specifically, by uncommenting the lines one by one and commenting out other @imports. It seems the be a matter of too many imports, rather than any specific one.

I'm not sure how I can test this theory. Any suggestions?

Hmm. Normally there should be sth. in the error.log, such as

Fatal error: Allowed memory size of XXX bytes exhausted (tried to allocate YYY bytes) in /[webhost_path]/script.php on line 123

Whats your PHP memory limit?

phpinfo reports memory_limit of 24M

There's nothing in the error log except:

[Tue Jan 23 15:51:44 2018] [error] [client 123.123.123.123] File does not exist: /hsphere/path/to/domain.com/default_html, referer: http://domain.com/about
[Tue Jan 23 15:52:13 2018] [error] [client 123.123.123.123] File does not exist: /hsphere/path/to/domain.com/About
[Tue Jan 23 15:52:21 2018] [error] [client 123.123.123.123] File does not exist: /hsphere/path/to/domain.com/About
[Tue Jan 23 15:52:47 2018] [error] [client 123.123.123.123] File does not exist: /hsphere/path/to/domain.com/contact
[Tue Jan 23 21:26:26 2018] [error] [client 123.123.123.123] File does not exist: /hsphere/path/to/domain.com/data/_cache/jquery.min.map;
[Tue Jan 23 21:26:41 2018] [error] [client 123.123.123.123] File does not exist: /hsphere/path/to/domain.com/data/_cache/jquery.min.map;
[Tue Jan 23 21:27:14 2018] [error] [client 123.123.123.123] File does not exist: /hsphere/path/to/domain.com/data/_cache/jquery.min.map;
[Wed Jan 24 00:49:31 2018] [error] [client 123.123.123.123] File does not exist: /hsphere/path/to/domain.com/data/_cache/jquery.min.map;
[Wed Jan 24 01:07:48 2018] [error] [client 123.123.123.123] File does not exist: /hsphere/path/to/domain.com/More, referer: http://domain.com/
[Wed Jan 24 01:07:48 2018] [error] [client 123.123.123.123] File does not exist: /hsphere/path/to/domain.com/default_html

I've enabled Report Errors in the Administration Panel but that shows nothing. I guess next step is to enable gpdebug and gpdebugjs in gpconfig.php?

phpinfo reports memory_limit of 24M

24 MB is not sufficient.

While Typesetter will normally stay below 10 MB for regular page serving, the Scss/LESS compilers need more.
Is there any chance to increase it?
To at least 48, better 64 MB.

edit: with only 24 MB you might also run into issues with resampling and generating thumbnails of larger images.

My hosting provider has kindly increased memory_limit to a whopping 512M, and problem has been solved.