Professor Barclay won't call back!

Hint 1 of 6:

This seems to be a common problem and I think I have the answer.

Hint 2 of 6:

There is a certain item in the Ludwig Museum at Herrenchiemsee that triggers
Professor Barclay's phone call.

Hint 3 of 6:

Did you spot Ludwig's diary?

Hint 4 of 6:

It *is* a bit difficult to see.

Hint 5 of 6:

It's located in Display Room #2 and it's to the left of the paining from
Grace's dream.