Thanks - I am not using the latest version, but just downloaded the update (am just a little nervous about updating, haha). I will certainly do so before I dig into the problem further. Thank you for telling me where to find the tables ... I will see if I can figure that out.
I do believe it was a problem with the custom fields b/c it started happening right after a created an event with a custom field.
Since then, I have deleted that event & custom field. I also upgraded to the latest version of DT Register (2.6.2), as well as rechecked my PayPal settings (IPN was enabled, but messaging was not. Perhaps that was part of the problem as well).
In any case - it seems to be working now. Thank you for the help!