In old instant films, the image you see appears directly where the film was exposed when you took the photo. The wide portion of the border contains developing and fixing chemicals. After the image is exposed, a roller breaks the chemicals free and spreads them across the film. Based on the consistent appearance of the splotches, the rollers are probably damaged and developer is not being spread across the film evenly.

They are unlikely to be a film defect because of their appearance across multiple frames. They are unlikely to be a defect in the lenses or other parts of the camera because of their repeating pattern down each frame.

Although I did not find any tutorials online about fixing the rollers specifically, the following resources may be helpful:

• Fujifilm Instax 100 Disassembly
• YouTube: Fuji Instax 210 torn down (explains how internal mechanisms work)

It looks like the rollers need to, at least, be cleaned. If they are pitted, they probably need to be replaced. (You could try filling the pits with resin. Then sand them down.) I wonder whether, a dowel could be used to spread the chemicals around after the film has been ejected from the camera.