Did you mean ether2-master-local as local bridge?
Is there a problem at the ISP side or do i need a different configuration?
Yes, that's what he meant. However.....
With SLAAC you get only an interface address.
This is absolutely correct. Furthermore, Mikrotik does not support assigning an interface address using SLAAC.
The ISP needs to route at least a /64 prefix to you.
They can do it by either....
1) A static configuration on your ppp profile on their end - If they do this, then they'll tell you what your local prefix is, and you can just assign it directly to your ether2-master-local interface.
2) dhcpv6-PD (prefix delegation) - this is Mikrotik's preferred method of receiving dynamic IPv6 prefixes.
It's obvious that your ISP doesn't support option 2 right now because the client failed to obtain any prefixes. You could also try putting the dhcpv6-client directly on the ether1-gateway interface, but it's very doubtful that this will work.