My experience for what it is worth. I had a 125 ticket flying IB that didn't post. After 2 weeks I tried submitting a missing avios claim and just like you I got the error message. I rang an agent who got me to upload ticket and BP's to this address
https://www.britishairways.com/en-gb...ting-documents
Later the same day I also contacted someone via chat on the website. 6 days later tier and avios were credited. Not sure which method was ultimately successful.
I would really love to know from someone here who is well versed in IT systems, as to why BA issued 125 tickets generate errors when trying to claim missing avios. Good luck with getting yours credited