Adding reference to Solver in Excel messes up the reference path

[obaluse]

That's right.

There is no missing text. Microsoft support is not right on this one.

However, I think in this case the problem might be something else, highlighted by this comment:
If you haven't included/referenced both files, that could possibly be the cause of the error.

Apparently I can only add the .xla as reference. It doesn't seem to accept the .dll.