Thumbnails appearing as white squares is a known issue that may affect media imported from Apple Photos on certain models of Apple iPhones. The Mylio Photos team is working to fix this bug in a future update.
In the meantime, to resolve the issue, the media must be removed and reimported. This can be done on a computer by disabling Safe Delete, moving all of the affected images out of their current folder (in Finder/File Explorer), then moving them back.
Open Mylio Photos on your computer
The computer must have a Device Sync Policy of Vault or be connected to an external drive vault
Open the Mylio Photos Dashboard and select Devices > Your Device Name. Scroll down and toggle Safe Delete off
Right-click the folder in Mylio Photos that contains white images and choose Reveal in Finder/Explorer (if your computer is a vault) or Reveal in External (if your external drive is a vault)
Move the files to a location that isn't part of your Mylio Photos library. This could be your Desktop, the root of your external drive, or another location
Please note that all associated files should be moved together. This may include files ending in HEIC, MYB, XMP, and/or _display.jpg
Wait for the photos to disappear from the Mylio Photos application
Close and reopen Mylio Photos
In Finder or File Explorer, move all of the files back to their original folder. The files will be reimported into Mylio Photos and a new Thumbnail will be created.