Error message "500 - Internal Server Error" or "0 - communication failure" displayed in the Theme Manager and/or no themes are displayed.
When opening the Theme Manager all themes and their extensions are processed by Shopware. The more themes and extensions exist in the Theme Manager, the longer and more demanding the process.
This error usually refers to unsufficient server resources. Most of the time, you can find the exact reason in your PHP errorlog on your server. In case you are not sure where to find it, please contact your host.
Your errorlog wil most probably provide indications of your PHP configurations "memory_limit" or "max_execution_time". If so, please raise the setting to a higher value. In case you don't know how to raise the value, please contact your host.
If the errorlog doesn't display any error message or if the error still exists even after the adjustment of the PHP value, you should raise the Ajax timeout. Please choose "Configuration > Basic settings > System > Backend > Ajax timeout".