I cannot tell you exactly which combinations would work. But from the CLI of the PS TS you could look at:
set user .... The user command has some control sequences for breaking out of connections.
set user name=root ?
If the above gets you nowhere, you can open a case with Digi Support. You can do this by sending an email with details to email@example.com
The problem description in detail, product, serial number, and firmware revision would all be helpful to include.
Digi Technical Support Engineer