I encountered this error after trying to install Duplicati 18.104.22.168 on a Synology NAS: it wouldn’t start in the GUI, and when running from the command line I got the above in amongst a lot of other errors. As admin, I had first installed Mono from SynoCommunity, then manually installed the Duplicati .spk from Duplicati’s GitHub (in retrospect I might have been better off going to the download page, which I presume links to a stable release).
After installing strace from entware-ng I pinned the problem down to none of the Duplicati package’s .exe files having their execute bits set. Fix, after ssh’ing in as root, is:
find /volume1/\@appstore/Duplicati -name \*.exe -print0 | xargs -0 chmod a+x
… after this, I could run the Duplicati service from DSM, and subsequently start the client from the main menu (the above may depend on entware-ng versions of find and xargs; not sure).