Finally i solved it… Adguard blocked it 
As written. Fresh installation Alpine-Linux (on proxmox vm), Docker, (Portainer), Heimdall.
No matter of portainer or pure docker, or if i remap ports or not.
i always get this error either when i click directly on “Add an application here” at top of the empty screen, or when clicking “add” after entering the application list.
Any idea? pls tell me what further information you need.
Edit: the funny thing is, i used the same steps for a earlier installation and it worked for some time, then suddenly the error appeared… deleted the VM and started from scratch with no success
Edit2 : just tried the same on debian-vm … same error
Edit 3 solved: cURL error 7: Failed to connect to appslist.heimdall.site port 443 in the logfile ← Adguard blocked it, now