I have exactly the same issue. Something went wrong. Only solution is to delete the cookie in this browser. Other browsers work at the time with same login info. No go until I delete this:
(xxxxxxxxxxx is a 16 byte number).
Pie Register registration form works at the time login does not btw.
Tried to delete caching, disabled caching plugin completely, no go unless cookie deleted. This is with fully updated wp, woocommerce and latest pie register free version with math captcha enabled. Let me know if you need more details from me. This issue happens every few days. I log in many times during each day. Hope this helps to track down the issue.