S3 and x1 backups get stuck on prepare phase due to the amount of files that are attempted to be open at the same time:
2021-04-26 8:50:16 0 [ERROR] InnoDB: Operating system error number 24 in a file operation. 2021-04-26 8:50:16 0 [ERROR] InnoDB: Error number 24 means 'Too many open files'
Adding "--open-files-limit=100000" fixes the issue.
This didn't happen on Stretch, so it must be a new behavior of innodb for 10.4 (or a bug). While s3 contains a lot of files, I would expect xtrabackup to run almost serially (unlike on normal database execution). Maybe a new optimization leads to file descriptor exahustion.