[I know this was discussed back in 2018 but I am currently having same issue and after cleaning the filter, drain hose, etc. I was warned of the below issue...
I cleaned washer with vinegar (without clothes), then again with only water]
The F21 error code SOMETIMES alerts to indicate "to much suds/wrong detergent".
Which leads to excessive suds causing the error code.
The washer should automatically fix after a long repetitive cycle...
Errorcode continually flashes until it fixes the sud issue, which is....
automatically drains, rinses, & repeats... until excess sud nonsense is resolved
**the code will reset after the washer resolved issue
Also [non-spill bleach] can be the cause of your "excess sud" issue.
Good luck, hope it helps