Hi Dave,
I have set memory_limit = 1024M in both php.ini and .user.ini, which as expected, has not changed anything.
Yesterday I installed myBB on a separate directory and ran a merge procedure to migrate from SMF to myBB. The merge was successful and everything was preserved perfectly... except that forum threads and posts are wrongly sorted. You have a post from 2010 following a post from 2018, then one of 2020, ... a complete mess.
I have no idea as to how to solve this problem. I want to migrate PixInsight Forum to a more contemporary and better maintained forum engine, but unfortunately I cannot find a reliable way to perform the required migration from SMF. At any rate, we cannot lose forum contents, since they form an important knowledge base and are the history of PixInsight. I'll keep looking for a solution.
Again thank you for your help and support.