AlricKyznetsov said:
As far as I know, the 500 issue was fixed for awhile.

For the second point, no. No other posts with that tag exist.

The issue was fixed for users that are privileged and higher. Privileged and up were to always still have the ability, just that if you are below that you would be barred from changing tag types via metatags and must rely on the tags page. However, a glitch early in this feature removal's implementation made it so that even privileged and up couldn't change tag types via metatags and everyone was relegated to the tags page for any tag type changes. It has since been fixed so that, user privileged and higher, can use metatags but lower users cannot and have to use the tags page.