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Magento 2 Backend Unable to login Service Unavailable

I am having a huge problem with my Magento 2.2. and I’d like to ask for your help. Frontend works perfectly, customers can login too.
I am unable to login to the Admin Panel. I can type the login details but after that I get this error message:
enter image description here

I made no changes in the website before this.

Things I’ve done so far:

  • check for var/maintenance (there was no file btw)
  • php bin/magento indexer:reindex
  • php bin/magento cache:clean
  • php bin/magento setup:di:compile
  • php bin/magento cache:flush
  • php bin/magento setup:upgrade
  • There is no error message in debug.log, system.log
  • Created a new admin user: same result
  • checked the server usage, not even 10% (storage, processor, brandwidth etc.) was used in the past 7 days

Many thanks in advance.