Hi Julen,
Does this happen all the time or only with a specific conversation?
If it's only a specific conversation, at what point in the conversation does it happen?
If it happens at the start of the conversation, make sure the <START> node's Dialogue Text and Menu Text are blank.
There are also updated UI scripts in
this post. There's a remote possibility that they might help. Back up your project before importing the package, just to be safe. These scripts will be in version 1.7.7.2.
Please feel free to send a reproduction scene to tony (at) pixelcrushers.com. I'll be happy to take a look.