I can confirm that its the inline evaluation (the = after the assignment) which causes the error. Contrary to the error message is has nothing to do with the labelling ("Beschriftung") of the variable.
This inline evaluation does not make much sense as it would juts repeat the row vector which is assigned. Nonetheless IMHO it SHOULD work and so I would say that you found a bug in Prime. You may report it to PTC support.
Here a simple example demonstrating the bug in Prime 9:

Its a new bug as it worked OK in Prime 6

I don't have Prime7 installed so I cannot tell if the bug is already present in this version, too, but you report it in P8 and its still not fixed in Prime 9.
EDIT: Just noticed that you wrote in the subject that it worked OK in Prime 7. So the bug was introduced with P8 and it came to stay 😉