Hi
I have just upgraded manually by overwriting the existing files with no problem, but that was from 7.1.5 so if you have a good copy of the old database reinstall that with 6.11.7 and try a manual upgrade.

@francoispl has pointed out to me, more than once, that it's not a good idea to manually alter the database :)

isn't the first time that I alter the database manually and never had a problem, but if a script return an error o a sql query fails something not work as intended and need to be modified.

In the first post I added an inline image but the site generated an error and not added... i try to add the image here:

I tried to upgrade another site (not the one that generated the error), with a little database, but in this case I tried to upgrade to a an old version of 7.x.x (7.1.4 more precisely) and after upgraded to the latest one with no issue, but I want to reiterate and underline that the database of this site is really small since there are 4 posts and 6 images (the backup of database il 350Kb), instead into the site that generated the error the database backup is very huge it contain 10 collection and contain post of lasts 6 or more years ( the database backup is 15Mb) so probably the problem resides here and the migration is impossible.

now before to try a new attempt I waiting the weekly backup of my hosting service so in case of issue I'm covered.