Thanks to Salamander, I've had a look at the Hamipiks code and discovered a significant bug in the code which determines whether an image has been updated since last time you looked at it.
This bug was causing an Internal Server Error after a PHP upgrade. The bug was in my code, not PHP, but was only tickled with a newer PHP version.
The interesting thing is that pressing Refresh caused the internal server error, which then meant the browser didn't try to send an 'If-Modified-Since' header, because the internal server error forced the browser to remove the image from it's cache and so forced a full fetch. Thus, the symptoms were that every second refresh attempt would work.
I've now fixed the bug and hopefully Hamipiks should start working properly again. Please let me know if it does not!
Sorry about that!