BIGNEWY
Think I've found the cause. When DCS loads up, Windows10 pops up a message from the right saying something like "Press Wins + G to record game etc".
When DCS loads, the logbook link is about where that pop up is on the desktop.
If I ctrl+alt+del out of game and choose task manager, then alt+tab back to DCS, logbook link works.
Basically, that pop up is messing with the menu me thinks.