Sharepoint Access Services - forms failing to load

I have an Access app under development published to Sharepoint. The app contains 3 data forms and an unbound text form. Until two days ago everything worked correctly, but yesterday when I logged on to Sharepoint the main Navigation Form loaded correctly, but when I select any of the three navigation tabs the corresponding forms fail to load in NavigationSubform - instead I get the error "an error has occurred".

Since nothing in the app or the data had changed, I undertook the following diagnostics:

I first verified that a plain unbound form containing text only (no data or macros) opens correctly in NavigationSubform.

I then removed the source query from one of the forms that would not load. With the form not bound to a table or query and the fields all unbound that form also loads correctly.

I then restored the source query to the form, but bound only one of the fields to the query, leaving the rest of the fields unbound. The form does not load.

I then removed the source query and bound the form directly to one of the tables. Again, the form does not load.

I then tried a second form, which is bound to a different table/query – like the others, it loads correctly when unbound from the recordsource, but fails to load when bound.

It looks like there is some problem with the data on the server. Any suggestions would be appreciated.
Thanks
Dave