[imp] logging in to IMP (as initial_application) fails

[Silver Salonen]

Hi.

I've set in Horde's prefs.php that the initial application should be IMP.

Now when I login to Horde, sometimes inner frame of IMP shows me login-form 
and says that "Login failed."
When I check my maillog at the same time, I see that user logs in 
(successfully) and logs out right after that. When I click on Mail in Horde's 
sidebar, IMP logs in successfully and shows me my mails.

When I log out of Horde and log in right after that, I get the IMP's "Login 
failed." form again.

I wouldn't like to mess my users' heads with that false error. How could I fix 
it?

Thanks in advance,
Silver