Can't upgrade: CSRF attack warning, then error about major versions

Ok, found that info in your logfiles:

As referenced in the FAQ Updates between multiple major versions and downgrades are unsupported within your github post this is an unsupported upgrade as you have skipped one major version (8.1.x). A correct done upgrade would be:

8.0.3 → 8.0.14 → 8.1.9 → 8.2.7

by following the steps provided in the FAQ above. The important part is that you delete all files and folders except /data/ and /config/ before extracting the new files of the next version you want to jump to.

Further reading is also available in the official documentation: