Saving records to connected tables

Hi, I have a working MS Access application. Now I am in the process of moving the tables to MariaDB 10 on a Synology server and use Access as a front-end. But there is one problem I could not yet solve. When I use a form for entering data into table A with a subform to enter data into table B and linked by a common key (primary key in the form with a reference key in the subform, I cannot enter any records ("Cannot save record to table B"). Has anybody a solution to this problem?

Thanks and best regards, boz