Invalid CIDR, but it's not

we do not allow bigger subnets than /8
why do you want this?

as a workaround you can put in 2 networks fc00::/8 and fd00::/8
 
yeah but it seem that is only relevant for fd00::/8

The block fc00::/8 is undefined. It has been proposed to be managed by an allocation authority, but this has not gained acceptance in the IETF.
from https://en.wikipedia.org/wiki/Unique_local_address

also, again you can put 2 trusted networks (fc00::/8 and fd00::/8) in and it should work
 

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