RSE Communications Server hangs

All of a sudden WDSC (5.1.2) hangs when trying to communicate with my main 
system. I have to use Task Manager to kill it.

Symptoms:
- When I try to execute a command on the iSeries (V5R2), WDSC says 
"Connecting to RSE Communications Server" and then "Initializing RSE 
Communications Server" and the progress bar starts filling from left to 
right. Nothing happens; clicking the red button to terminate it just 
changes the colour of the button.
- On the iSeries I can see a job QZRCSRVS. The last entries in the joblog 
are a number of "Client request - run program QSYS/QUSRJOBI".
- When I try to execute the same command on another iSeries, everything is 
fine. The QZRCSRVS job also has a number of QUSRJOBI entries, but 
continues with "Client request - run command 'CALL     '".
- When I try "Verify connection" I get an empty pop-up screen and it 
hangs. When I do the same to another system the pop-up screen fills with 
messages and the OK-button is activated.
- I get the same symptoms when trying to connect with WDSC 5.1 from 
another PC.

Yesterday everything was fine, so apparently something in the iSeries 
changed last night, but I have no clue what it might be. I've asked 
around, but noone seems to know what happened. JDK 1.4 was installed this 
weekend; last IPL was Monday morning; complete backup was made Monday 
evening.

Anyone know what to look for?

Joep Beckeringh