Nobody’s quite certain exactly what causes this. Microsoft’s explanation of the error is, “The menu handle was invalid.” Thank you very much, Microsoft.
It happens a lot, relatively speaking. Every bittorrent client has reports of it, as do most other internet applications including Eudora, Lotus Notes, &c. clear back to the introduction of NT, with nary a satisfactory explanation or solution.
There are some workarounds that seem to help, but that’s all that’s really available. One thing that sometimes works is to always save the torrent to a file on disk, then load it into your client to start downloading. Another is to clear out your temporary internet files folder.