Dude 3.6 cannot connect to newer versions of ROS.
In recent (well not so recent) versions of ROS the authentication method has changed, so old Dude cannot connect anymore.
AFAIK there is no workaround except to update.
But, while updating might sound cool, you make sure you do tons of tests and backups before actually migrating to v6.
v6 solves just as many bugs as it introduces. And it's pretty much a dead project again. So any introduced bugs in v6, you'll have to live with them for a long long time before (if) they ever get fixed.
So... unless you really need to be able to connect to ROS via Dude, I'd say, stay on v3. I know I've done so (on my largest installations of Dude).