You are passing a seq[string] in tp.spawn processor(buffer). I do not know if buffer is being copied into the call or passed by reference or whatever, but that seems potentially troublesome.
Furthermore, it looks like you are using status-im/nim-taskpools, which, in the readme states:
The following are non-goals:
I actually don't quite know what that is saying (does it want or not want thread-local heaps?), however I think it basically means don't pass any GC-ed type like seq or string to spawn.
With the above in mind, this may be just not a correct use of the lib.
My guess is you could do move buffer and delete the = newSeq... line as something to try.