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R3.0.0-rc17 for RPi does not allow to upload a backup (.zab) file from the Management interface
The "Upload the file" button is always greyed-out

Uploading a .zbk file via ExpertUI does work

Just in-case you missed this.
you need to select the box "Are you sure you want to overwrite your current configuration? Have you created a backup?"

Yes, I have selected both boxes

oh well, wait for support to answer.
They always say clear the browser cache :p

I did clear the cache (twice), used two different browsers (Firefox and Chrome), but still the same issue.
The debugger console of the browser reports an error "TypeError: angular.element(...).scope(...).checkSelectedFile is not a function", after I select the file.
I have tried this on upgrade from 2.3.8 -> 3.0.0 as well as a fresh install from V3.0.0
RPi 3B, z-way-server RaspberryPiXTools-v3.0.0 on rasbian Stretch (but tried Jessie too, no difference).
After re-installing V2.3.8, the backup upload works fine (after clearing the cache), re-installing V3.0.0-rc17 brings back the problem

We will make sure it works in next rc