UPDATE
Just got this from BetFair:
Sorry for the delay and thanks for your patience.
The issue was caused by a defect introduced by a recent API release.
The defect resulted in the INVALID_PRICE error when a bet was updated to any of the following values.
2.26, 2.28, 2.3, 2.32, 4.1, 4.6, 5.1, 8.2 and 9.2.
The release that introduced this defect has now been rolled back. We have fixed the issue and will make the release again later this week.
Excellent news! It seems like the previous problems are now sorted!