It seems that I managed to make a tag that behaves strangely (#total//effect). Presumably judging by that regex in the error (Expected "tagSlug" to match "[^\/#\?]+?") tagged search doesn't like the slash.
Curiously, though, it works fine as a profile-specific tag:
Not sure what the remedy here is. Don't allow tags with illegal characters to be created maybe? I don't know if that's the intention even, haha.
It looks like this got solved, thanks so much!