We are not aware of a known issue which may cause this problem. On the account which fails, does the proxying user have the same rights to it as other accounts or for users where the applet works fine?
If you proxy manually to the account, does the applet (which normaly fails) work after a manual proxy is performed during the same session?
What is the exact error message?
The reason for the question is that we have noticed some strange behavior in the past when proxying to accounts via an applet. We found that while a manual proxy would work for a target account, an applet proxying to the same account would fail with a ‘user not found’ or something similar. It would display a strange user ID, one which did not relate to the actual account name to which the applet should proxy. We eventually solved the problem by tracking down and deleting a Personal Address Book contact that the error had referenced. Somehow the proxy via the APIs was trying to resolve the account User ID against another entry in a Personal Address Book.
Perhaps it would be worth reviewing the address book issue and maybe run an OFCHECK against both the target and source accounts.
I hope this helps in tracking down the cause.