Error: pthread_self() called from alien thread named 'Broker Thread' no RM attachment (READ pf_addr=0x4b8 pf_ip=0x10f62dfc from pager_object: pd='init -> manager' thread='Broker Thread')
Looking at thread.cc, it seems you're subclassing Genode::Thread and (presumably) instantiating it using the Genode thread API.
Others will correct me if I'm wrong but I think as of Genode 19.x it's no longer possible to call the LibC from such "pure native" threads.
You'll have to use the POSIX API instead (pthread_create() etc), and threads created in such a context will correctly register themselves in libc.lib.so and no longer trigger the assert() in pthread_self(), in the ubiquitous errno handling and so on.
Cedric