An error occurred while collecting items to be installed
session context was:(profile=DefaultProfile, phase=org.eclipse.equinox.internal.p2.engine.phases.Collect, operand=, action=).
No repository found containing: osgi.bundle,org.eclipse.mylyn.commons.ui,3.18.0.v20151116-1930
Eclipse finds no conflicts, but at the last moment of Check for Updates, I get the above error. I don't think mylyn used by the BLESS plugin(s).
Hopefully, just adding an Available Software Site will cure the error message.