Ok...I got that error.
It is definitely related to the thread title. As I understand VBulletin, tags are generated automatically for the title of the thread.
I tried to post a text thread in the Canning and Preserving forum. My original title was "ISO TNT Pineapple - Habanero Jam", which resulted in the exceeded maximum number of tags by 1 error.
I edited the title to read "Need TNT Pineapple - Habanero recipe" and received the same error when I tried to preview my post.
I then edited the title to read "Need Recipe" and was able to successfully preview my post. I did not submit the post.
Somehow the tag generator is generating too many tags, as I have not found a way to manually add tags, although there may be a way to do it. I will see if I can try a few more things to see what triggers this, because it doesn't do it all the time.