A production source tells us there have been allegations of inappropriate conduct by Langella made within the last couple weeks on the set of the limited series "The Fall of the House of Usher." We're told there's currently an investigation into what happened, but Langella has not been fired or suspended from the series.

As for what exactly happened, a source close to production tells us the 84-year-old actor allegedly made an inappropriate joke that was sexual in nature. Our sources also say in the context of his performance, possibly during rehearsal, he touched the leg of a female costar, and further drew attention to the action when he jokingly said something like "Did you like that?"

The show has been filming in British Columbia, Canada ... we reached out to the local police department who says nothing has been reported to them involving Frank.

Our sources say the investigation has not had any immediate effect on production, as Frank -- who plays Roderick Usher in the series -- was not scheduled to shoot any scenes this week.

Langella found massive fame as Dracula in 1979 and more recently played the judge in Netflix's "The Trial of the Chicago 7."

We reached out to a rep for Frank ... so far, no word back.