"An internal server error occurred. […]" during plugin installation

Internal server errors (error 500) are often caused by plugin or theme function conflicts, so if you have access to your admin panel, try deactivating all plugins. If you don’t have access to your admin panel, try manually resetting your plugins (no Dashboard access required). If that resolves the issue, reactivate each one individually until you find the cause.

If that does not resolve the issue, try switching to the Twenty Sixteen theme to rule-out a theme-specific issue. If you don’t have access to your admin panel, access your server via SFTP or FTP, or a file manager in your hosting account’s control panel, navigate to /wp-content/themes/ and rename the directory of your currently active theme. This will force the default theme to activate and hopefully rule-out a theme-specific issue.

If that does not resolve the issue, it’s possible that a .htaccess rule could be the source of the problem. To check for this, access your server via SFTP or FTP, or a file manager in your hosting account’s control panel, and rename the .htaccess file. If you can’t find a .htaccess file, make sure that you have set your SFTP or FTP client to view invisible files.

If you weren’t able to resolve the issue by either resetting your plugins and theme or renaming your .htaccess file, we may be able to help, but we’ll need a more detailed error message. Internal server errors are usually described in more detail in the server error log. If you have access to your server error log, generate the error again, note the date and time, then immediately check your server error log for any errors that occurred during that time period. If you don’t have access to your server error log, ask your hosting provider to look for you.

I manually reinstalled WordPress by deleting the SQL, deleting the WordPress files via SFTP and then re-uploading the default wordpress files with a new configured wp_config.php – so everything should be set to default settings and default plugins.
If I upload an XML backup of all the posts I’ve written before the error appears again. So I think plugins and settings problems are already checked as the error appears in a completely stock WordPress environment only having a recovery of the blogposts…

For my previous tries (personal data has been hidden):
“[Mon Jul 11 14:20:57 2016] [alert] [client $CLIENT] /var/www/virtual/$USER/html/.htaccess: Option All not allowed here, referer: https://$DOMAIN/wp-admin/plugin-install.php?tab=search&s=broken+link”

The .htaccess only consists of WPs default entries and an addition to redirect every http call to https. and some redirection rules to redirect some other permalink structures to /%postname%

All of the sudden, it somehow works now – without ever doing something additional… Strange…

Well, that’s good to hear! ??

For the people googling around and resulting in this page:

steps I’ve done:
-reinstalling WordPress via SFTP (using another table_prefix in wp_config.conf)
-recovering my posts from a previous data-export via tools -> data export (import via tools -> data import)
-increasing the php.ini memory_limit value to 256M (even if 64M should work too)
-cleaning the servers .htaccess of all stuff not inside # BEGIN WordPress and # END WordPress

Hope this will help you even if I can’t say which action was it to make it work 2 days later…

Thanks for sharing that!