I just upgraded to 3.4.8 and the agents I had previously set up
no longer work. If I try setting them up the way they were - I still don't get
a connection. Can this be symptomatic of having 3.4.8 and the older version on the PC at the same time?
Or is it something else?

Which version you ran before?
What is the exact error message you get when connecting to an agent?

: I just upgraded to 3.4.8 and the agents I had previously set up
: no longer work. If I try setting them up the way they were - I still don't
: get
: a connection. Can this be symptomatic of having 3.4.8 and the older version
: on the PC at the same time?
: Or is it something else?

: Which version you ran before?
: What is the exact error message you get when connecting to an agent?

We ran 2.4.15

Error Message: Connection to remote host failed - connection refused.

Agents are working in 3.4.8, but you've got to do 2 things for the this upgrade:
1. Upgrade all agents and master schedulers.
2. Enable and configure security on each agent (Tools/Security menu)

: We ran 2.4.15

: Error Message: Connection to remote host failed - connection refused.