Hi. This has happened to me twice.
I was adding a bunch of tags for K2 using the tags page (not while editing an item). Suddenly, my admin panel gives 404 page. I think it's because of a space character within a K2 tag. Could it be so? Can I delete tags manually, since I don't have access to joomla admin page?
Joomla version is 3.6.
Can you help?

*EDIT*
This lasted for half an hour or so, and now it's back. Frontend was fine all the time. Nothing in error.log. :(

It could be a server or database hiccup.
You can insert spaces in tags and delete them manually.
Was the 404 error through all of K2's admin views?

Yes, in all views. So spaces in tags are not problematična?
Is there a way to delete or edit tags without admin panel access?

You can delete them though the database, but the admin is a safer choice.