For me, it was the fact that essentially a 7.5 language pack was in use that dashboard.php couldn't do anything with. If or when I have some free time, I'll just update those language files myself.
Rename the Dutch "admin/login.php" language file to "admin/dashboard.php"
There are a few new lines at the top of the languages/english/general and languages/english/admin/general that you'll also want to move over to your 7.5.x custom language files. They are related to versioning.
Ah, thanks. It appears I too made this mistake as my custom theme had some colors declared in this class (long before the upgrade). Corrected now!