I recently upgraded a couple of internal wikis from mw1.16.5 directly to mw1.20.2.
Everything was working when I noticed that uploads were not working anymore. As I already knew I had to correct the rights in the Linux directories so thatthe upload would be working again (images direectory). As always this helped and the uploading was working again.
But on one wiki it did not help. After looking into the problem for a couple of hours I changed the mysql database name by coincidence from "w.wiki1" to "test-wiki". Suddenly the uploading was working again. I then found out that as soon es there is a "." in the database name uploading would stop working. The "." was not a problem in 1.16.5.
For me this is resolved as I won't use a "." in database names any more but I still wanted to point out the problem.