The problem on my client’s site was the same, along with NO images loading on any pages until you clicked on the page a second or third time. I found that the problem was caused by Responsive Lightbox & Gallery plugin’s incompatibility with the new version of WordPress, 5.5. The plugin I mentioned above – Enable jQuery Migrate Helper – has solved it for now, but the plugin needs to be updated to be compatible with WordPress 5.5 So a question for the plugin’s developers – are you working on an update? It’s urgent that this be fixed for my client’s site on a permanent basis. Thank you.