Between 20% and 40% of the time when I log in I get this error, and have to log in again: I never blocked cookies for ET. I have the impression it happen if I push the enter key too quickly.
Try a different browser and see if you get the same error. If you don't, then you may need to clear all cookies for our domain, as one or more of them could be corrupted.
Wouldn't I have it then all the time? Problem started a few weeks ago. I use these settings in my browser already several years. If I get the error, I just log in again without touching/changing any settings. Would the problem not occur again if it was a cooky blocker? If I log in slowly I never have a problem; if I log in fast I get the error.
What do you mean by logging in slowly/fast? EDIT: I think I understand now. You literally mean logging in s...l...o...w...l...y etc.
You haven't mentioned much (OS, browsers, vpn, hard drive type, other problems, etc.); but this is the first link I've found that lists issues similar to yours. https://forums.developer.apple.com/thread/89050 It also explained to me what you mean by 'logging in slowly/fast.' A post at this link states that the problem can 'go away' after a short time period. That may explain why it works for you when you log in slowly.
No, slowly means after typing the password I wait 2 seconds before hitting the return key. If I hit the return key fast after I typed the last letter from my password, I get the error. Firefox, vpn, ssd. Never have/had this same problem on any other site. Not a huge problem as I just wait 2 seconds and the problem is solved. I just posted as I was thinking it might be a minor website problem that could be fixed.
Cool. You may want to run something like CCleaner anyway. (It'll clean a bunch of stuff, including cookies--but you can specify which sites you want cookies to be left alone, if you like.)
I do CCcleaner, adwcleaner, spybot and glary utilities on a regular basis. Have a list of cookies that should stay.