Hello dbs, as always thanks a lot for your help. The problem is exactly the same, only this time I didn't upgrade to version 2.10 but to version 2.8.3 rev 1645 and the wysiwyg module stopped loading. I wanted to upgrade this account little by little as a test before I upgrade the account that really worries me which had some code injected.
This account was using PHP 5.4 and I changed to PHP 5.6 after I tried to upgrade it and received a message that I needed PHP 5.6 or higher. I was able to solve the wysiwyg module loading problem by switching to PHP version 7 instead of 5.6 without having to upload all the files again. I don't know why it didn't work with PHP 5.6, I am guessing I needed to switch to version 5.6 BEFORE running the upgrade script the first time?
Anyway, I am going to proceed upgrading to SP7 now and then to 2.10, hopefully it all goes smoothly.
Regarding the other thread, I had totally forgotten about it. When I checked it again, it was working. I have no idea of how it fixed itself!