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I searched around a bit in the forum but found nothing relevant. I would perform a database backup in ACP and got a message.

What exactly does it mean? What should I do to measure?

if your web host panel has error log then check it for latest entries. if it has no clue then contact your web host

Ok, thanks

A 500 error usually indicates something has gone wrong server side or application side. As .m. suggested I would start by looking at the error logs as this will give you a good indication of what's gone wrong.