Occasional redirect_to reauth=1 problems

I’m having a frustrating issue on a large Wp/Bp Ms install (originated in Mu in 2008) where users will occasionally get the ?redirect_to=http%urlstuff&reauth=1
?redirect_to auth=1 when going from the front of the site to the dash, regardless of path. The logged in cookie gets dropped.

There is no discernible pattern to this issue… so it’s not stopping anyone’s work so much as annoying folks.

I’ve deleted all plugins (including a multi-network), no change. I’ve turned off SSL login, no change. I’ve turned off caching, no change. I’ve futzed with the htaccess a but nothing has changed.

Anyone have other ideas for troubleshooting?

tia…