Cannot change added tag

Hi,
After a while from posting a post, now I cannot remove a tag and add another one. For example when I remove the tag A (by clicking the circle) and add the tag B, after updating the post, nothing’s changed and the tag A returns without the tag B.
I can remove it and update the post, then it is removed. But when I add a new tag, the old tag returns without the new.
The new tag contains ‘+’ character but when I try another tag (doesn’t contain ‘+’) it works.
What’s the problem with the ‘+’ character?