This most likely points to SD card corruption rather than normal camera files. A bad or failing card, a counterfeit card with fake capacity, or less likely a reader/computer issue can all produce unreadable folders and odd large files.

Best practice:

• Back up every readable image immediately.
• Don’t just delete the strange entries and keep using the card.
• Reformat the card in the camera before reuse.
• If the problem returns, retire/replace the card and consider testing it for fake capacity.

Formatting in-camera is generally recommended, ideally before important shoots. If you recently changed to a cheap card reader, it’s also worth trying another known-good reader to rule that out, but the symptoms still strongly suggest filesystem/card corruption.

If the card has been in use for years, replacement is sensible if you care about reliability.