This sort of thing is usually caused by program updates that introduce bugs, or the updates cause malformed code/expressions to no longer work. If it's happening on all pc's check the build and versions of Access on them and if recent, try rolling the update back on one and see what happens. You could also open the query in design view, copy and post the sql and maybe someone will spot something that's not quite right.
The more we hear silence, the more we begin to think about our value in this universe.
Paraphrase of Professor Brian Cox.