Well, Muldaur called the experience a 'complete nightmare' in a 1999 interview, so it might be true.
I've also heard the director of Unnatural Selection saying these things as well. Muldaur was a terriffic lady, according to him, but the rest of the cast treated her horribly (Stewart as well).
I've heard this rumor too! Many of the cast were very rude to her during her time on TNG. I know that Patrick Stewart was very vocal with his displeasure over Gates McFadden's firing and openly campaigned for her return.
I've noted two things in the blu ray that lead me to believe that the cast wasn't very fond of Diana. Michael Dorn was the only to mention anything positive about Muldaur. Marina Sirtis said that she had nothing in common with Diana that she was married and went home to her husband whereas she had fun with Gates and Crosby off camera. Patrick Stewart didn't even mention her. And Muldaur tiptoed around it but said she had decided that she would never work with people she didn't like regardless of the script or the money. This could just have been editing but the TNG crew is a cast that goes out of its way to constantly remind us how much they love each other. It's very telling that Diana is very rarely referred to.