[7210] PC Suite 5.1 -MESSAGE SENDING FAILED- error

The problem:
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The Nokia PC Suite 5.1 (wich I'm using for the 7210) refuses to send sms-messages. When sending, the error "message sending failed" appears, and the message is moved to the Outbox, where it stays. Note that my speakers do make that funny sound when the phone is establishing a connection, and sending the text.

I connect through InfraRed (USB), and I can do everything with my phone through the software on the computer, except send sms-messages.

Strangely, everything worked properly before I re-installed my computer. I haven't changed any setting in the phone, that may result in this error.

What I've tried to solve it:
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- Re-install the PC (i'm an it-professional, so that should be ok);
- Check the message-centre number on the phone;
- Remove the country-code (+31) in the message-centre number;
- Re-use the country-code in the message-centre number;
- Check numerous sites for a solution to this problem.

The specifications:
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- Phone: Nokia 7210;
- Provider: KPN Netherlands (subscription);
- Software: Nokia PC Suite 5.1 (fresh copy downloaded);
- Platform: Windows 2000 Professional SP4 (EN);
- Connection: InfraRed (SWeeX IR USB device).