Comment # 14 on bug 1208161 from Michael Hirmke

I found the reason:
Besides /run/systemd/generator/systemd-cryptsetup@root.service there was
another systemd-cryptsetup@root.service file in /etc/systemd/system.
The difference was

/run/systemd/generator/systemd-cryptsetup@root.service:
BindsTo=dev-disk-by\x2duuid-47c6783a\x2dd2b0\x2d4740\x2dabd3\x2d4f0f43f63d8c.device
After=dev-disk-by\x2duuid-47c6783a\x2dd2b0\x2d4740\x2dabd3\x2d4f0f43f63d8c.device

/etc/systemd/system/systemd-cryptsetup@root.service:
BindsTo=dev-disk-by\x2duuid-47c6783a\x2d2b0\x24740\x2abd3\x24f0f43f63d8c.device
After=dev-disk-by\x2duuid-47c6783a\x2d2b0\x24740\x2abd3\x24f0f43f63d8c.device

i.e. the latter contained a wrong escaped guid for the encrypted partition.
I have no idea, what created such an escape sequence and why this file existed
at all.

After removing /etc/systemd/system/systemd-cryptsetup@root.service, the problem
vanished.