Queries with a single filter failing

I am performing a queryEntity, with no other information than pagingInfo: topaginginfo(1, 1), and a filter on the field "letter", checking it is equal to "a".

In my database, "letter" is either "a", "b", "c", "d", or "e". The query works with "b", "c", "e" but fails for "a", "d".

What are the possible causes for this query to fail?

 Discussion posts and replies are publicly visible