After I have enabled
FileVault the computer won't boot anymore at the scheduled time. Is it normal or has something gone wrong? If it is normal, are there any workarounds?
FileVault enabled, the disk is encrypted and cannot be accessed at all without you entering the system login password. If it weren't so (e.g., to make unattended boot work), then the point of
FileVault, which is preventing access to anything without your explicit authorization, would be lost.
So yes, it is normal. You could create two disk partitions, encrypt the non-boot partition, and put your data there. Or, you could create encrypted disk images and use those for data you want encrypted. The problem with these approaches is that it's very hard to keep potentially sensitive data from leaking into the non-encrypted part: caches, swap files, backups, etc. If you simply encrypt everything, then this management headache goes away.
It's a classic tradeoff between security and convenience.