ACF post-object relation on register-form not shown

Hi,

i just migrated to the newest version 7.1.5 and start migrating all forms from 6.4.16.

I am using an ACF “post-object” to let the user select a value from these posts on user-register-page.

In the old version, everything worked and the ACF-fields were shown and filled on the register-page.
Now, ACF fields are shown, but the select-box is not filled with the post-objects.

Any suggestions?
I would like to get this working before buying further extensions for TML.

Thanks in advance.