Dear all:

Today I did an auto-upgrade System>Maintenance>Auto Upgrade
And ran into the same trouble as I have had before all over again.
However, this time I was able to fix it myself.
With my provider, the 500 error screen suggests that there are either badly set file permissions, or a problem with .htaccess.
If your site was running properly before, .htaccess ought not to be the problem, however, file permissions on the install folder are; at least in my case.
The install folder had today - as before - the wrong file permission settings: 775 instead of 755
For the problem to go away, you have to set the Install folder permission to 755 and all the file permissions to 644.
The easiest way to do this is through CPanel > File manager.
Once done, you can reload yoursite.com/install/index.html (or index.php) and will probably get a 503 error (site in maintenance)
This happens because the install script gets aborted, so the site is not taken automatically out of maintenance mode.
To take the site out of maintenance mode you need to rename the file umaintenance.html in the conf folder to _umaintenance.html
After that, you ought to be able to access the install folder and run the index.php script to update the DB and finish the install without further problems.
Hoping this helps (at least someone)

Cheers,
Gerard