Hi,
the linked app is explicitly stating:
Minimum required ownCloud 8.1
Maximum required ownCloud 8.2
As 8.0.4 is outdated (you're currently missing 12 updates as 8.0.16 was the latest version of this branch) and unsupported since the last month i'm closing here. Please upgrade to oC 8.1 or higher. A few info about this is available here: https://central.owncloud.org/t/upgrade-to-9-1-on-qnap/3803
Please get in touch with the QNAP support for more support.