Hello Bjarne,
I have taken a further look at this issue and yes, I can reproduce this behaviour - this is a current limitation of external environments.
If you're triggering the external environment on a table, updates against the same table are not currently permitted - it is very easy to create a deadlock in this situation.
Furthermore while researching your issue, I discovered that attempting to create a new SAConnection from the SAServerSideConnection information will crash the server - I have opened this investigation as CR #764064.
If you need to make a secondary connection, you should create a new SAConnection from scratch with your user credentials and use that connection to perform updates (if you absolutely have to update the same table - but be very careful about deadlocks).
Regards,
Jeff Albion
SAP Active Global Support