@dubird can you check the error logs of the server?

The copy of the files themselves is fine

This is usually the main cause of this kind of errors during upgrades. So, if you're sure all files are correctly uploaded to the server, then we need to find another reason for the error.

Is it possible for you to upload a fresh copy of b2evolution into a new folder at the root of the web server and try to load http://yourdomain.com/newfolder ?

Test directory works. I finally found a backup that I was able to get working, so I'm going to try updating again, this time manually instead of through the dashboard, see how that works.

Ok, manual update worked. However, something odd. I used 6.9.3, the latest stable release. It's giving me the error "b2evolution version 6.8.8-stable-2017-03-09 is now available. We recommend that you upgrade to the latest version. Download here." Not sure why it's telling me to update to an earlier version.

@dubird please make sure that the latest version of the file conf/_application.php is also uploaded to the server. You should find the following line inside:

$app_version = '6.9.3-stable';

I actually found this error when some files permission settings were wrong.

@achillis wrote earlier:

I actually found this error when some files permission settings were wrong.

Me too: see
http://forums.b2evolution.net/repeated-upgrade-failure