Don't use capital letters for your system users :D
A very sad story who happened to a friend of a friend:
Connecting a SAP CRM and an RP 2005 to a PI 7.0 I became fool with an RFC destination.
Testing the same RFC destination in RP 2005 and in the CRM; in RP the connection was fine, but in the CRM it does not worked at all. I tried to change the password many times, delete and create the connection, but it didn't work.
When I was considering the autoinmolation against the servers I finally fall in the possible mistake: my CRM is not the newest one,it had an old version and wasn't case sensitive. My password had capital letters. So that's why it worked fine with the RP(The newest RP and quite case sensitive) and not with my poor and old CRM.
Then, the moral: Don't use capital letters for you PI system passwords above all if you connect with some old Backend...