@Chan - If you are using Chrome, right click anywhere in page and inspect element. It will fire up a developer console and you can see under Network tab, see any errors. If files missing it will report it.
Do you mean to check for errors/missing files this way after I (re-)attempt the upgrade to v8.00.21 and it fails with blank screen? Or do this on current v7.02.07 before upgrade to see if my v7 install has problems that are causing the upgrade to die?
For many years our users have been locked up with such issues upgrade and files server environment. The developers here will solve this soon by letting us doing the upgrade remotely. I have started the roadmap for it and hopefully we will see daylight in Babylon. That is if you opt in for Version 9 later.
I am hoping to jump to v9. I have been checking every so often to see what the touted "final" release would be but am rushing to v8 in the meantime since staying on v7 is scary, because PHP7.x has made ongoing compatibility so sketchy.