You are sure, that this error did not occure before you updated your forum?

No, I was reluctant to update because 2.4.18 was working good but I decided to update because it seemed only a couple of things had been changed like

fixed: when editing a posting, an activated subscription can not be saved in the database because of a syntax error on the database query

I've never seen a 500 error on MLF before and I guess this forum is on version 2.4.19 as since I updated to this version I too get 500 errors.