[imp] Problem logging in for a specific user

[Pierre-Luc Soucy]

Hi,

I have a user whose mail can be successfully downloaded in a POP3 client 
like Mozilla, but Webmail fails to login using the same login details.

In the logs, there's a login success message for that user:

Aug 29 21:02:10 HORDE [notice] [imp] Login success for [...]

but on the web-based interface, it shows the following error message: 
"Login failed for some reason. Most likely your username or password was 
entered incorrectly."

The webmail system works fine for everybody else, but not for that 
account, although the login details used are valid and tested in other 
e-mail clients.

Any idea what may be wrong? Where should I look to get more information 
about the problem?

Thanks in advance,

Pierre-Luc Soucy