A possible explnation is the SOLMAN is the CUA master. This means that the RFC user in it's own logical system is the "call back" user but you are expecting it also to perform the CUA client tasks (for itself).
CUA "hogs" the logical system names with the same name as the technical connections, but the master is a child of itself.
--> Check the auths of the user in the logical system of SOLMAN and your SCUM settings.
--> Check for programs doing DB updates on USR02...
--> Check for locks on the user in Sm12 (this is also a common cause of confusion, as you are locally in change mode (lock) and the RFC user in logsys wants to change it as well...)
ps: There is a setting in table USR_CUST via which you can relax the locks, but you should actually fix the problem....
A good solution is an IDM and only maintain identities from there.
Cheers,
Julius