I’ve just got only 303 in my log:
POST /login HTTP/1.1" 303
And then login page reloading.
Whats wrong with the app?
If you mean Nextcloud by “the app”, I can only say that there is nothing fundamentally wrong with it, because I and millions of others who use Nextcloud do not experience this error 
Seriously, this is impossible to answer without more information. And since you decided to ignore the support template and did not provide any information about your setup and what you did before this error occurred, I doubt anyone will be able to help you 
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