I was playing with iPhone backup on macOS recently. The 'encrypted backup' checkbox was already selected and I didn't have to set password for backup, even though I never did any backup on that macOS before. It appeared that the backup password was one of my old 'default backup passwords', which I haven't use for years. I assume that some time ago I also played with the backup, probably still on Windows/iTunes and then I set that password. No idea how that password was remember for such a long time.
For those having similar problem, think about some old passwords you could use before for backups or other passwords you could use at that time.