install any plugin via WPMU DEV Dashboard it returns "500 Internal Server Error"

The issue is:

when you try to install any plugin via WPMU DEV Dashboard it returns "500 Internal Server Error" (red error bar at the top)
there are also 500 Internal Server Errors in browser console for admin-ajax.php calls

What's been done so far:

WP_MEMORY_LIMIT was raised
some "clean up" of "wp-config.php" was done (it was a bit messed up)
I reviewed server via cPanel but I can't see anything unusuall there
caches were flushed multiple times both from WP admin and from server; I even disabled cache on server but that didn't help too
Somewhere along the way, it was possible to install Hustle via WPMU DEV -> Plugins but then it caused entire wp-admin to go down upon the activation with 500 Internal Server error. Other plugins still return that error upon any attempt to install.

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