Hi Christian and all, I'm experiencing a weird issue with a RestXQ service of mine which was working and suddenly starts having problems because it just stops serving requests. I know about SSCE and so on but this time it's hard to reproduce. Shortly, it receives about 5 simultaneous requests and for each of those it calls one xquery:fork-join (5 functions) to query another service. I cannot say what the error is exactly during the time when it is stuck. What I can see on the client are, besides HTTP 503 and Read Timeouts, the following logs which appear frequently but with no particular order: ERROR: Unexpected error: java.lang.NullPointerException: Cannot invoke "org.basex.util.hash.WeakTokenSet$WeakTokenRef.get()" because "this.keys[index]" is null (hint: /...y/webapp/nubisware.muscle.fiber.ccp_admin/ccp_worker_sk.xqm@65:33) ERROR: Unexpected error: java.lang.ArrayIndexOutOfBoundsException (hint: /.../webapp/nubisware.muscle.fiber.ccp_admin/ccp_worker_sk.xqm@65:33) ERROR: Unexpected error: java.lang.ArrayIndexOutOfBoundsException: Index 8 out of bounds for length 8 (hint: /.../webapp/nubisware.muscle.fiber.ccp_admin/ccp_worker_sk.xqm@65:33) Line and column numbers point always to the same xquery:fork-join call. Can you point me to what I should investigate? Am I hitting some quantitative limit on concurrency? Connections? Thank you for any help because I'm currently wandering in the darkness :-) Regards, Marco.