I ran into the same issue a couple years ago ... ASE 15.7, RS 15.7.1.
The issue came up shortly after creating a new repagent/connection into the repserver; most of the time the first 'transaction' through the system was a rs_ticket and just like clockwork my DIST threads would go down with the same error.
I did open a case with tech support and the gist of the issue [don't quote me on this] was that there was an initial hiccup with a memory allocation within the DIST thread which automagically fixed itself once a) a transaction came in from the PDB (and triggered the error) and b) the DIST was restarted. The workaround/fix was to issue a 'resume distributor' command.
I was in the process of migrating a handful of repservers from HP to AIX ... to include moving something like 200 database connections (across all envs) ... and having to run repeated tear-down/setup tests ... net result is this issue happened so often that I added a 'resume distributor' command to my thread monitoring script ... and then added a call to said script as part of the setup test. I don't recall any DIST threads going down again (for this same reason) once I had run 'resume distributor'.