Hi Beat, since your question is very specific and fairly technical, I've moved it over to Jive Support. I think you'll find more help asking your question in that space.
Hi Beat Zimmerman,
As Libby mentioned in her comment, this is a fairly specific scenario and a bit out of scope of a generic public discussion answer due to the amount of variables involved in an external storage configuration. As such, I would recommend submitting a case in your customer group ( MyJive Secret Support Group Administration FAQ ) as a next step, where a member of our Support team can assist in reviewing through your instance's configuration. When submitting the case, if you could include a link to a file exhibiting the behavior this should assist with their initial investigation.