BigNate469 wrote:

(#3)
Right-click/long tap (depending on the platform you're on) on the thing you want to put in your backpack, click “export” (which will save it to your device's files), go into the project you want to have it in, import it (usually the upload button in the "new “ menu, and usually on the top. For example, to import a sprite, click/tap the ”new sprite" button, and then click the upload button).

Note: this doesn't work with scripts, but it does for sprites, sounds, costumes, and backdrops.

It won't fix your backpack, but it does provide a temporary alternative.

wait until <scratch team fixes it>