Can not always login my webmail.

[voltaq]

Hi,
Sometimes I can not login my webmail.
I got the error that my username or password where not correct. But I am 100% sure that they are correct.
To be able to login I do the following:
- SSH to my server as root.
- Edit /etc/php.ini
- comment the : zend_extension = /usr/lib/ioncube/Ioncube_loader_lin_4.3.so

- Save /etc/php.ini
- Login my Plesk as admin
- Restart my Apache
Then I can login my webmail. When this problem repeats I do the same things again but this time I uncomment the zend_extension = /usr/lib/ioncube/Ioncube_loader_lin_4.3.so

I've also tried to login as <user>@<domain> and as only <user>
I am using Plesk 7.5.2 for Linux and had updated everything.

Somebody familiar with this problem?

Thanks in advance,
Renato