Well i was celebrating to early.
Accessing Dropbox Cloud with CC works fine, but CC crashes in the middle of the download process of the metadata.
My Books are stored on an ext. SD-Card, read/write access to SD-Card is given by the OS. Seams CC can not write the metadata file onto the ext. SD-Card. There is no problem to access and browse the books on the ext. SD-Card with CC itself.
Now the big thing

For some reason the write
and read access to the external SD-Card gets revoked/lost for the whole system or other apps after the crash of CC.
I have to reboot to gain access again.
I sent you crash reports via CC.
Strange!
Just made a test exporting files from the Dropbox application to the ext. SD-Card. No problem at all.
Running:
Samsung Galay Note 10.1 with BlissPOP ROM (CyanogenMod 12.1 based ROM) - Lollipop 5.1.1 - rooted.