Windows vs. Linux

Hello,

I recently moved my application to a Linux based hosting company from a Windows based provider. Now, I have noticed the following:

When users fill in submit a “profile” document from a web form that contains a UID field, some of the documents get created with the UID set just fine, others have a null UID field. Those that get created without the UID are unable to log in after registering. Any ideas why some docs are missing the UID???

Thanks in advance,

Jan