So I was redirected to the list to discuss http://tracker.firebirdsql.org/browse/CORE-4467.
What I can add is that it occurred to me to run a validation on the database after I get the error (I keep a copy, just in case backup/restore goes wrong).
Result is :
Summary of validation errors
Number of record level errors : 3
Number of database page errors : 1
Given that backup/restore goes smoothly, I assume the errors are in metadata pages (somewhere in index if the error is anything to go by).
I`m willing to run any suggested tests that might help. The load that breaks the database is reproducable but I`d really prefer not to upload a few years worth of customer data (worst case scenario - I`ll scramble it and take the chance).
As I said before - up to about a month ago things have been running fine.