Hey Support! Thanks for nothing! :-(
For all others with the same Problem , here is the solution:
1. find ../plugins/system/jacontenttype/models/types/your_migrated_k2_extra_field_group_name.xml
2. edit the xml file, and change for all not required fields - required="true" to required="false"
Sample:
<field
name="ctm_hp_fax"
type="text"
label="PLG_JACONTENT_TYPE_HP_FAX_LABEL"
description="PLG_JACONTENT_TYPE_HP_FAX_DESC"
default=""
required="false"
/>