All was working fine and then upgrades killed the site which was running MarketPress and GridMarket.
After deactivating all plugins, the conflict is between MarketPress and GridMarket.
Only using a different theme allows MarketPress to run.
With only the MarketPress plugin active, then activating GridMarket results in white screen of death.
I rolled back to older versions of MarketPress, Frame Market and GridMarket and they get along fine with each other, but somebody among the new versions is not playing nice.
Upgrading results in White Screen of Death.
Can somebody please figure out the culprit and fix it?