Airbag control problem

Hello, I've been having a problem with the airbag warning light for some time now.
Last week, the airbag warning light wouldn't turn off. I tried to read the error memory of the airbag control unit using VAG-COM. The error memory didn't show any errors. After that, the airbag warning light was off again, and it worked normally for about a week. Today, the warning light is on again and has been constantly illuminated.
Can anyone tell me what might be causing this, or has anyone else experienced this problem?
Here are my vehicle details:
Audi 80 1.9 TDI 1Z, manufactured in December 1993, B4 model.

Best regards,
Hans