The tag route was recently renamed from tags to tag. It seems likely that the issue is being caused by a plugin.
Can you try going to your site’s error logs (found at Admin / Logs / Error Logs) and then tagging a topic? Possibly this will trigger an entry into the logs. If it does, or if you see any old log entries that refer to tags, have a look at what is displayed in the ‘info’ and ‘backtrace’ tabs for the log entry.
The ‘backtrace’ tab might give us information about what is causing the issue. It will look something like this:
