I tried HTTrack again and got it to download all my PMs, so was able to delete everything in the forum.
Now I can receive messages again.
If anyone is interested, I needed to log out of the forum, fill in the username / password to log back in, and then use the URL capture function in WinHTTrack before clicking on the button to log in. After logging in again in my browser, I used the URL capture function a second time switching to the 'My Messages' page. In order to keep the program from downloading the forum, going through the user list, etc, you need to setup a Scan Rule to exclude URLs containing 'ubb=' and then after that, setup an include filter for URLs containing 'ubb=viewmessage' (which will get the PM index pages as well as the individual messages). You may want to exclude .zip and .exe files to avoid downloading programs you recommended to others (or vise versa), especially if you check off the option to get non-HTML files related to a link (ie for avatars); you can also set size limits, etc. I set the maximum external depth to zero so the program would not download other webpages, and left the maximum mirroring depth un-set to make sure it went through all the index pages. I think the only other thing I changed from the default was to set the html footer to 'none'.
When the program is done you may have a list of errors (due to broken links from old homepages or avatars, etc). If you leave the Build option set to the default of 'site-structure', you may want to sort through the folder structure WinHTTrack creates and delete any empty folders or those containing just html error messages from an expired avatar or screenshot link. There are also options to store html files in one folder, and all images in a specific subfolder, which may be neater (the way the forum links are done, the html files all get put in the same folder anyway).