Hi Arne,
with your terrific help I got it hopefully solved now. 
Now I'm able to load data.
Obviously it seems that I need only ECECUTE privileges for my schema to get access to my tables within my container. I removed all privileges for my tables.
This is now my role which solved my problem.
.hdbrole
{
"role": {
"name": "TBASE.db.roles::cdsTablesRole1",
"schema_privileges": [{
"privileges": ["EXECUTE"]
}]
}
}
.hdbroleconfig
{
"TBASE.db.roles::cdsTablesRole1":{
"Ref1":{
"schema":"285BH6G95SZF188B_TBASE_HDI_CONTAINER"
}
}
}
Thanks a lot and thanks for your patient with me.
And for the others, don't forget to grant the role, as above described.:
GRANT "285BH6G95SZF188B_TBASE_HDI_CONTAINER"."TBASE.db.roles::cdsTablesRole1" to _SYS_REPO