You could try the following:

1. Increase the shared pool size.

The Oracle Debugger seems to have some stability issues when there is not enough free memory in the shared pool.

2. Disable the call stack.

Go to Tools > Preferences > Debugger and disable the "Update call stack after each step".

3. Check for user trace files on the server.

It may be that the server process has died. Check if a trace file exist that is related to the problem, and if so, send it to me.