Does your error handler have a generic response for any error that you have not foreseen? If so, are they tripping it and are you logging the error number, description and as many of the variables as possible? This may be a permissions issue or an unanticipated user approach, either of which might cause a variable to be null or empty. I say that because it appears as though you're saying it happens to others but not to you. Secondary might be your break option in the vbe editor, such as "break on all errors" rather than just unhandled ones.
The more we hear silence, the more we begin to think about our value in this universe.
Paraphrase of Professor Brian Cox.