Just a minor note of clarification on this: it won’t install, but if you had Duplicati installed before the “upgrade” to DSM 7 then the service still works if started manually (you might have to go through the whole TrustFailure bad X3 cert rigmarole again though). Since it has never worked by automatic start for me anyway, I have just manually restarted it. Consequently, if you’re only running the beta then you may well have no problems until the next beta comes out (since DSM 7 won’t even recognise the 2.0.6.3 installer file now).
Manual start is detailed elsewhere, but the key things are giving it a good big temp folder to work with, and making sure that your server data folder isn’t anywhere that’ll get wiped by a DSM upgrade; basically, put both into a volume you control rather than on the DSM root:
TMPDIR=<a_good_big_tempdir> \
/volume1/@appstore/mono/bin/mono-sgen \
/volume1/@appstore/Duplicati/Duplicati.Server.exe \
--server-datafolder=<your_duplicati_data_folder>