Corrupted phone contacts after syncing, after selecting default phone number using the Symbian OS

I've been using Soocial for a few weeks now, and up until now (apart from a minor configuration problem) without any problems. But now I've found some very annoying behaviour, which I will try to describe in detail in the hope that it can be corrected.

The Problem
My phone runs on the Symbian OS. When a contact has more than one phone number, one can select the default phone number to dial. However, when using this, some of my contacts loose some data in my phone, like first name and part of the last name. It does not happen with all contacts, and always with the same ones. I suspect that it has got something to do with special characters or the length of the last names, but to be sure I should do some more testing. The data on the Soocial server remains correct. When altering the data on the Soocial website, thus forcing a sync, the names appear correctly on my phone again, but the default number which I selected earlier is no longer selected. So I seem unable to select the default number to call on my phone, and that bugs me. A lot. I did some checking, and the problem seems to be that a contact is marked as "changed" when I select the default number on my phone, therefore send to Soocial when syncing, and then synced back to my phone (!) with corrupted data. That last part was a surprise, it makes no sense to me.

Phone Specs
I am using a Nokia N95 8GB phone with firmware version 20.0.016, and a Dutch version of Symbian 3rd edition. I can't update my firmware because Vodafone has not approved a newer version.

And Now?
I haven't found mention of this problem on this forum using the search option, so I decided to report it as a problem. I can do extensive testing if you require more data on this. I'm fluent in Dutch and almost so in English. I hope you can confirm this bug, since I can think of nothing I can do right now to fix this on my own.