Locus GIS portal

Hi, I created a shape file using ArcGIS on my PC with several columns. Some of them filled, some of the columns empty. I now want to use Locus GIS on my smartphone (android 16) to fill in empty values of this shape. Thus, I imported the shape into Locus from the cloud. Here, I can open and see the features and edit the shape. However, when I want to fill in a value into an empty field, I get the warning messages that I have to fill all empty attributes (even though, they are explicitly defined as not obligatory in the settings - see an example attached). Even when I fill all empty fields, the warnings disappear, however, I cannot safe the editing. Same, when I fill all empty fields e.g. with a dummy value using ArcGIS and import it again, Locus refuses to safe. Does anyone know a possible solution for this issue? I'd be thankful for any advice. Best regards, Toni