Comment # 13 on bug 1016313 from Michael Woski

(In reply to Franck Bui from comment #12)
>  1/ to replace:
> 
>     ExecStart=/usr/lib/systemd/systemd-timesyncd
> 
>     with
> 
>     ExecStart=/usr/lib/systemd/sleep 10000
> 
>     You'll need to copy /usr/bin/sleep to /usr/lib/systemd/
>

works 

>  3/ make the service as simple as possible by removing all fields in
> [Service] section that are not needed to reproduce the issue.

well, it's the @mount option in the SystemCallFilter. Removing all other
properties does not help as long as that @mount option is set

>  4/ show the output of "strace -c <content of ExecStart>"

see attached files

> Also it might be interesting to see what happens if you add
> "SystemCallErrorNumber=EPERM", I meant if the debug log show something more
> interesting.

it's working, but I would have expected that.

> BTW could you describe your affected setup ? is it a virtual machine ? which
> arch ?

x86_64, "real" machine, openSUSE Factory