I've encountered this myself.
It seemed to be when the GM initiated the contact, I was in my regular world. But upon entering an instance (in my case, Eureka Pagos) the situation changed. The GM could continue to send me tells, but I could only ever reply once. Every time I got a message, alt+r to send a reply worked, but only for a single message. Any attempt at following up was giving me errors about how I can't send tells in duty.

I hope my little tale regarding this helps them fix this.