We've had this happen to 2 different routers now. I did a Winbox drag-dop upgrade with routeros-7.15.2-tile.npk from 6.49.13. I don't know why I didn't go stairstep through 7.12. Upgrade seems to have converted my configuration correctly as OSPF routing and firewall features seem to be working correctly. Winbox initially loads and then disconnects before the session loads completely. I seem to have found that the fix is to either delete the winbox saved session or to load session <none> and overwrite it.

The more you know...