Upgrade NAV 2009R2 to NAV 2017 - Login Failed

Hello,
I am trying to upgrade a database from NAV2009R2 to NAV2017, but this keeps failing for some reason I cannot figure out.
This is what has been done so far (following the
- Database is tested (no errors)
- Upgrade601700.BE.1: data transfer is done + objects are deleted
- Open database in NAV 2013
- First time 'altering objects'
- Data conversion is done
- Second time 'altering objects' runs to 100%
- Third time 'altering objects' stops halfway with a 'login failed' error.

On SQL server an error is logged: "Login failed for user xxx'. Reason: Failed to open the explicitly specified database 'Upgrade2009R2'. [CLIENT: 10.1.1.19]"
On the client nothing is logged in the event viewer.

Some more information
- All tables compile before the update
- All tables linked to a view are deleted (linkedobjects=Yes)
- All statistics are deleted
- Compatibility level is correct
- NAV2009 version = 6.0.0.33793
- NAV2013 version = 7.0.0.36374
- Database = +/- 150GB. Available space on sql server = 500+ GB
- We have 40.000.000+ GL entries, so I use the trick of Vjeko (http://vjeko.com/how-i-reduced-data-upgrade-time-by-78-hours)

Does anyone have an idea? Any help will be greatly appreciated.
Andy