They are seeing the normal phpBB message that the file is too large. And the size limit that they are seeing is the old limit (100M). I don't think it's a timeout issue. If it was somehow a timeout issue that resulted in this error message being displayed to the user, it should at least say that the limit is 250M. I do not know where the 100M size is coming from. I have changed my php.ini file to 250M. Even the regular dreamhost default php.ini file is not 100M, it's 7M.
One thing I'm wondering about is my site was recently moved to another server by dreamhost. I don't know if this could have anything to do with it.