Photos app for Mac showing some blurry photos.

do you still have the original iPhoto Library, that you migrated to Photos? and do you have iPhoto9.6.1 installed, do you can check, if the iPhoto library still has the photos, that are now blurred?

to check, if the originals of the blurry photos are still in Photos, try to export them with "file export > export unmodified original". That is safer than copy and paste.

"Unsupported Video Format"... What is going on with Photos????

Which version of MacOS X is running on your Mac?

The iPhone X may be taking photos in a format, that oilier versions of MacOS X are not supporting. If your iPhone X is using the space saving HEIF/HEVC format, you need macOS 10.13 High Sierra to view them, see: Using HEIF or HEVC media on Apple devices - Apple Support

If you can see the photos correctly, when you click them to view them enlarged, try to rotate them back and forth, to force the creation of a new thumbnail.

• Select one of the empty thumbnails.
• Then enter the key combination ⌘R to rotate the photo.
• Then enter ⌥⌘R to rotate it back.

If this brings the correct thumbnails back, select several photos at once and rotate them this way.

I tried to search the file by its name on the Contents of the Photos Package, but it shows me only the 0kb files...

If the images are still in your Photos Library, they should appear in the Masters folder

Ctrl-click the Photos Library, use "Show Package Contents", then open the folder Masters.

You should be seeing folders "Year > Month > Day ..." of the date of import and if you click through the nested folders the original images should be there.

Where is your Photos Library stored? Is it in your Pictures folder or on an external drive?

Are you using iCloud Photo Library?

Have you been using any cleaning applications to remove duplicates?