Login after anonymous search leads to unknown error

As the topic describes:
When I try to login after a topic search while I am not logged in, then the first login leads to the response in the login window "Unknown error".
An immediate click on the "login" button then leads to a successful login.

I have no such problem when using Firefox 69.0.3 (64-bit) or Internet Explorer 11.239.18362.0 on a desktop Windows 10 Enterprise x64 18362.239

Same constellation over here, still it happens:
Perhaps some more steps are involved:
Browse the forum on page "Latest".
Use the search.
Open a linked hit page.
Copy the URL of that page.
Return to the main page latest.
Click on the topic that caused the search in the first place

What do you mean by return? Switch back to a tab that contains Latest? Is the user to open a desired page from Search in another tab in the first place? Or do it all in one tab, going back and forth with web addresses?

Yes, just one Tab.
I think it has to do with the cookie that saves the previous search and then the login.

I has so much cookies today that my tommy hurts

I spent almost an hour an a half with he online help from my bank proving [futilely] to them that they and only they have some glitch in the code after recently implemented changes that affects the login process. Long story short I told them I do not have any problems with login in whatsoever whenever. Mp3tag forum is no different- I did what you described and did not get to see that error. I performed this test with cookies being blocked and after cookies cleanup done in the middle of the day

I suggest to wipe clean yours and block them as well- and see what happens

The cookie stuff should work - always.
Here is another sequence:
click the magnifiying glass
enter "traktor4"
see the list of topics.
Select one of the entries
Copy the URL
Click on the MP3tag forum header to open the main page
Click on latest
click on the topic with Custom traktor4 (Custom tag "Traktor4"...?)
Click on Login
get the error

And yet my 1 of my banks managed to mess this up and plays stupid, wasting time of their customers, asking the same questions twice. [And I know I am right because after recent changes in the European law, after 5 minutes of inactivity the user should be logged off automatically- but being logged on to my account on their site if I bring up the side window with the chat room I can stay as long as want doing nothing, i.e. reading the current page that I were on when the 5 minute mark was hit. And what do they say to that- that I can rate the service of the chat room if I am not pleased with it]

I performed but got no error