PHP Deprecated: strpos(): Non-string needles will be interpreted as strings in the future. …

[esszett]

This Warning leads to a typo I guess:

MarkupProcesswirePhotoswipe/MarkupPwpswpGallery.module

Line 141 in a7c4959

if(strpos($resizeOptions['size'], 'x' === false)) throw new WireException("size option must contain \"x\"", 1);

… this should be:
if(strpos($resizeOptions['size'], 'x') === false) throw new

[jjozsi]

I can confirm this fixes the problem. Well spotted.