In Apk Pure → Update, it tries to install → Extract APK, give Signal icon and ask to Install an Update, and then Installation Failed.
Sailfish … Settings … System … Non-trusted programs …
Allow Non-trusted programs
I have this setting on. If that is not set in your device, maybe you can try to set it on and try again installing Signal?
One other thing you could try is to copy the install file to some other place and try install from there.
I had always On this setting.
Finally found it - Aptitude or APKPure failed to notify, that there is insufficient memory on internal storege (~250MB).
Only after going to Aurora Store, and trying to install, I have got top screen line, information, that shown me what is root cause of issues with installation:
“Signal private messenger insufficient memory space”
Also, when tried install as double tap APK file from File Browser, I get silent fail with no info on issues…