Update fails with no very clear error message

[probonopd]

me@host:~$ /isodevice/Applications/AppImageUpdate-171-b380930-x86_64.AppImage /isodevice/Applications/Firefox-latest.glibc2.3.4-x86_64.AppImage 
AppImageUpdate version 1-alpha (commit b380930), build 171 built on 2017-11-27 17:25:25 UTC
Checking for updates...
... done
Starting update...
Updating from Bintray via ZSync
open: Permission denied
zsync2: Failed to parse .zsync file!
zsync2: Reading and/or parsing .zsync file failed!
Update failed

[probonopd]

Checked with the latest version of AppImageUpdate but get the same:

me@host:~$ /isodevice/Applications/AppImageUpdate-185-1c72f3b-x86_64.AppImage /isodevice/Applications/Firefox-latest.glibc2.3.4-x86_64.AppImage 
AppImageUpdate version 1-alpha (commit 1c72f3b), build 185 built on 2018-01-03 16:41:48 UTC
Checking for updates...
... done
Starting update...
Updating from Bintray via ZSync
open: Permission denied
zsync2: Failed to parse .zsync file!
zsync2: Reading and/or parsing .zsync file failed!
Update failed

[probonopd]

-j just exits without printing anything:

me@host:~$ /isodevice/Applications/AppImageUpdate-185-1c72f3b-x86_64.AppImage /isodevice/Applications/Firefox-latest.glibc2.3.4-x86_64.AppImage -j
AppImageUpdate version 1-alpha (commit 1c72f3b), build 185 built on 2018-01-03 16:41:48 UTC

[probonopd]

Additional information:

me@host:~$ strings /isodevice/Applications/Firefox-latest.glibc2.3.4-x86_64.AppImage | grep bintray-zsync
bintray-zsync|probono|AppImages|Firefox|Firefox-_latestVersion-x86_64.AppImage.zsync

[probonopd]

Issue not limited to Bintray, GitHub also affected:

me@host:~$ /isodevice/Applications/AppImageUpdate-185-1c72f3b-x86_64.AppImage /isodevice/Applications/Audacity-2270e4c-x86_64.AppImage 
AppImageUpdate version 1-alpha (commit 1c72f3b), build 185 built on 2018-01-03 16:41:48 UTC
Checking for updates...
Fetching release information for tag "continuous" from GitHub API.
... done
Starting update...
Fetching release information for tag "continuous" from GitHub API.
Updating from GitHub Releases via ZSync
open: Permission denied
zsync2: Failed to parse .zsync file!
zsync2: Reading and/or parsing .zsync file failed!
Update failed

[TheAssassin]

I'm pretty certain this is a duplicate of #1.

[probonopd]

#1 is about elevating privileges when needed, this is about giving a clear error message when things go wrong.

[probonopd]

Sure enough, running with sudo makes it work.

[TheAssassin]

Okay, but they're related problems.

[parcelcat]

I'm receiving the same error message when using the latest version of AppImageUpdate to update itself:

Fetching release information for tag "continuous" from GitHub API.
Updating from GitHub Releases via ZSync
zsync2: Failed to parse .zsync file!
zsync2: Reading and/or parsing .zsync file failed!