Media Encoder can't export Timelines with proxy files

I am editing my videos on a NAS system with proxie files and attached offline media files.
But I cannot export these timelines in Media Encoder. I get a warning that there are missing media files (original UHD files), but I can export every single timeline directly from Premiere without any problems.

So I guess this is some kind of bug?
I am using an M4 Max Studio with the latest Premiere / Media Encoder version 25.2.3 / 25.2.