Hi Tom,
I suppose, your problem correlates with mine…
https://generatepress.com/forums/topic/error-after-update-to-1-3-23-strpos-empty-needle-in-media-php-on-line-1088/
because there is some issue with the customizer, too. Could you please have a look and confirm/decline, so we do not produce double work load for (GP-) Tom!?
Deactivating the premium plug-in: and the error is gone! But I need the Premium add-on!!!
Hope, (GP-) Tom can help us both!
Regards, Sven