lapi_init() failed with rc(400) after upgrade to 5.3 of IBM,9113-550

[ArjenV]

We have upgraded our IBM,9113-550 to 5L 5.3 (previously 5.1) to
accomodate some 3rd party libs we need to build our software.
Unfortunately this upgrade hasn't really come out the way we would
have liked. It seems that ever ince the upgrade our LAPI is broken
somehow, and errors like:

MPI-LAPI ERROR: lapi_init() failed with rc(400)
MPI-LAPI ERROR: lapi_init() failed with rc(400)
ERROR: 0031-309 Connect failed during message passing initialization,
task 0, reason: 2660-400 Internal communication error.
ERROR: 0031-007 Error initializing communication subsystem: return
code -1
ERROR: 0031-309 Connect failed during message passing initialization,
task 1, reason: 2660-400 Internal communication error.
ERROR: 0031-007 Error initializing communication subsystem: return
code -1
ERROR: 0031-300 Forcing all remote tasks to exit due to exit code 1 in
task 0

are a dime a dozen. We have verified our RSCT version to be the latest
filesets (2.4.12.0 and 2.4.0.0) and are pretty much at a loss what to
do next. Output of lslpp -L rsct.\* can be provided if necessary.