Hi there, not sure if this will be of help but I encountered this exact same problem on Linux Mint 20.1. The error message is very unhelpful, and I found this thread (amongst many others, all without solutions) while searching for more information about the problem.
The actual solution was hinted at here:
SSHFS – Too Many Levels of Symbolic Links – Vassox
The solution was to use the command line to open an ssh connection to the destination as the root user (or use sudo) which prompts to add the host fingerprint to the cache, to which you say yes. Having done this once, like magic, the automount will work as expected.
Not sure if this is a "bug" in systemd or not, but it sure would be awfully nice if the error message was somewhat more useful!
Hope this helps!