I checked FTP access yesterday before submitting the connnection details: It's working (it already worked for previous support requests). I just checked it again using FileZilla and it still works. However, I used saved password in FileZilla. I updated the connection details.
I reverted error_reporting setting in configuration.php back to default (I was aware of this change yesterday).
I applied the patch in image.php. I had to restart the web site in IIS Manager to get the changes live. The site is now working again. Thanks for the information.
Please provide such restoration hints before leaving it in such a state next time. I got several calls and messages that our site is down.
Since I got an information by another extension developer, I will upgrade our production site to Jooomla 4.1 rather sooner than later like our test site.