Created attachment 951663 [details]
can't add tags for the cases because the 500 error

Description of problem:
as summary.

Version-Release number of selected component (if applicable):
v3.8.12 on devel server.

How reproducible:
100%

Steps to Reproduce:
1.open a test plan with cases.
2.then click "Tags-Add tag" in the Cases tab for some cases.
3.then submit the tag to check whether the tag can be added.
4.then click "Tags-Remove tag"in the Cases tab for the cases.
5.choose a tag and submit to check whether the tag can be removed .

Actual results:
1.can't add tags for the cases because there are 500 errors
2can't remove the tags for the same reason.

Expected results:
both can add and remove tags for the cases.

Additional info: