Importing any panel template and deleting all of its tags will result in no longer being able to add tags using the "New Tag" in the Tag editor.
Even when not using templates, creating a screen from scratch with even a few tools and assigning tags to them (used 4 since it was quick) and then deleting all of tags in the tag editor without deleting them from the tools first will prevent you from entering new tags.
Deleting only part of the tags seems to not break things as bad.
Xpress does give a warning that you are about to delete tags that are assigned to tools etc... which is a warning to fix the tools or delete them.
I was able to recover the ability to add tags by deleting all of the panels in my test application and then the "New Tag" worked, I then was able to add the panels back and as long as I did not delete the tags that import with them everything works.
Deleting tags assigned to tools is never a good thing regardless of if it is Xpress or InteractX etc... The tools do not have actual tag names referenced in them. They have a pointer which is a number assigned to the tag by the tag editor and hidden from the user which is linked to the Tag list. Deleting tags will break this pointer and adding back a new tag with the same name will not fix it since the new tag will have a new index number since tag index numbers are not reused. It is similar to the index number in any database. Deleting the id breaks the table. Once a tag that was attached to a tool is deleted, the tool must be deleted or edited to a valid tag if it lets you enter one on the tool. Also, doing mass delete and add of tags rather than having imported tags overwrite the old tags of the same name results in the index number increasing far in excess of what one might expect. The tag ids in an application we received where over 41000 which is also an indicator of lots of mass deletion and importing of tags.
The way to prevent the error from happening is to never delete all of the tags related to a panel and if you want to change a tag assigned to a tool, change the tag then delete the old tag one it is no longer attached to anything.