Comment # 1 on bug 1134447 from Dr. Werner Fink

Ohmm .. I think this is more a bug in osc-0.165.0 ... as I see also

 isc build --download-api-only --root /abuild/oscbuild/SLE-15
 Certificate changed unexpectedly. This should not happen
 SSL Error: certificate verify failed

or

  osc build --download-api-only --root /abuild/oscbuild/SLE-15
--alternative-project SUSE:SLE-15:Update
 Certificate changed unexpectedly. This should not happen
 SSL Error: certificate verify failed

... only on Tumbleweed with osc-0.165.0 (isc is an alias: osc -A <internal
OSC>)
and this does work with osc-0.164.2 on Leap 15.0