Problem with DIBS payment from some servers

[kentacoder]

Hi
I have an issue with a web shop setup with Joomla 2.5.6 , VM 2.0.4 and DIBS payment plugin j25v_4_1_0 (Danske bank). It is working from our testserver (Debian 6, Apache 2.2.22, php 5.3.15, Joomal 2.5.6) but not from the production server in a web hotel (FreeBSD, Apache 2.2.16, php 5.3.10, Joomal 2.5.6).
The DIBS payment site detect an error and say that the MAC value is wrong. Then supplying the "data string" reported by DIBS error page to their test tool, it says OK and works.
The php function used in plugin to create "MAC" verification code is hash_hmac. Is there an issue in difference to OS, PHP, charachter coding (should be UTF8) or something else?