Export function in version 10.3

In a completely redesigned, quite complex application, I have the problem, that the export function does not work. Neither with formatted or raw values, multiple deactivation and reactivation of the function did not lead to any success.
The same application set up with version 10.2 works fine.
However, both applications access different databases (MySQL). In the new database for version 10.3 I use quite long field names, but that can't be the reason for this refusal to work?
Does anyone have similar experiences?
Update: Testing with a fresh database and an new project = everything is fine. It seems, something in my project stops the export-function......grml..