Carefully follow this guide. When you’re done, you may want to implement some (if not all) of the recommended security measures.

Thank you! I checked with various malware plugins and could not find anything peculiar. But one plugin came up with all the deleted plugins and sorted them into a “read/write errors” list.
Seems I have to wait until I get access to my ftp again. I hope I can find the problem.

I suppose it could be related to whatever your hosting provider is doing behind the scenes too, you might want to reach out to them about that.