Thank you, Gizmo and Ruben, for your suggestions.

Rolling back php creates a big mess and breaks a lot of things, so did not try that. Looked for but couldn't find any dot files in the styles directory.

Ultimately, I decided to take a chance. Created a zero-byte writeable file ubbthreads_1617365481.css.php that the upgrade script was looking for. Resumed the upgrade and it went through! After the upgrade I checked to see what happened to that file. Still the same zero byte file.

Anyway, things are good now.

Thanks again for your responsiveness. I appreciate it.