In order to determine if this is a caching issue, may I get you to clear your cache and cookies, please? I've included a couple of step-by-step articles below from Google below for you.
Once you've cleared the cache and cookies, can you try to please create a new tag? If the issue is still persisting, you may need to try it out in Incognito mode to see if you're able to create a tag.
Can you try those steps out and let us know how it goes?
Thanks for reporting back! This does sound like unexpected behavior.
I would encourage you to continue working with our technical support team on your open ticket as they have access to internal tools to troubleshoot this. I would recommend sharing a HAR file with them so they can take a closer look at this error.