HTTP requests start hanging after I try activating all plugins, and only stop when I manually disable the plugins via DB

Are you checking the slow query log in mysql or connection count? Just off the top of my head I would say mysql might be choking on something. Also, top can show you which processes are going crazy when this happens.

Here is a bit on the slow query log and different ways to access its information:
https://rtcamp.com/tutorials/mysql/slow-query-log/

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