Is your VoIP Monitor Server Service is running on the CRS Server. Normally you will get the error message you have mentioned if the Supervisor desktop is not able to login to VoIP Monitor Server, the service should be runnning.
The error message is not due to spanning configuration.
Are you using 2 NICs on the PC/server for Voice monitoring? The 3550 does not support spanning traffic on the same port the PC/server is using as its LAN port. With a 3550 doing the PSAN, you should have 2 NICs.
These are the paths to get to each CCX logs through CLI. They may be helpful if you are having issues accessing RTMT or downloading logs through it.
If you want to download them you have to prefix "file get " and you can add one of the options (re...