For everyone experiencing the problem that control buttons (play/pause, volume, full screen) won't show in some browsers, here is the cause:
Mediaelement.js uses SVG icons by default and PNG as fallback. If the webserver does not serve the correct Mime Type, some browsers will not decode SVG/Z correctly (but still try instead of using the fallback-PNG source). Fortunately we can fix this.
A) The easy fix. Add the following lines to your .htaccess file in the gpEasy installation root:
AddType image/svg+xml svg svgz AddEncoding gzip svgz
and see if it works (do a hard refresh of the page).
If this doesn't help or causes an error, we have to use plan B:
B) Replace mediaelement.min.css file in the plugin's directory:
If you downloaded the plugin from gpEasy.com and extracted the ZIP manually into the /addons directory,
the css file is here [gpEasy Installation root]/addons/MediaelementPlayer/mediaelement/mediaelementplayer.min.css
If you installed the plugin live from within your gpEasy website's Admin Menu -> Plugins -> Download Plugins, the CSS file is in one of your /data/_addoncode directories [gpEasy Installation root]/data/_addoncode/someRandomHash/mediaelement/mediaelementplayer.min.css
In this case you will probably have to change the file's permission to 777 first before you are allowed to overwrite it. On some webhosts you may not change the permissions (because the file was created by the webserver and your FTP user has not write permissons). In this case you will have to uninstall the plugin, download it manually from here and unzip it to the /addons directory.
Download this mediaelementplayermincss.zip
unzip it and and overwrite the one in the plugin directory with this version.
I currently don't know how common the problem actually is. I tend to change the css in upcoming versions to PNG only.
SVG is cool but … not cool enough to cause such hassle.