Renaming it back did it.
I renamed it because the file name was toooooo long and was spoiling my commands in terminal, and was unreadable. Such names come from long website titles, which are used as part of file name. Maybe the domain name should be used instead of the website title? So I just named it bigbackup.tar.gz.
I was under impression that the .tar.gz file will contain all information needed for restoration within it, so you can rename it freely to whatever you like, and following whatever naming conventions you have for organising your backups. To be fair, I find it counterintuitive that the backup consistency depends on the backup filename, same as it would be weird if a Word document couldn’t be read after it gets renamed.