Since you are seeing a 500 error you may want to check your webserver logs to see the full 500 error being reported. In a lot of cases the webserver is failing to respond and that error can give you more details on the cause. You may also want to try restarting your webserver.

For the database error, a lot of times it is caused by running out of memory, so checking your mysql logs will indicate OOM errors and verify if that is the case.

Hi NJW, thank you for your prompt reply. Appreciated!
However, I tried checking mysql logs it says, “2017-04-11T17:38:22.604644Z 0 [ERROR] InnoDB: Cannot allocate memory for the buffer pool”.