How do i regenerate thumbnails?

Pretty sure the thumbnails are created WHEN it creates a database. I needed high res thumbnails so I made the thumbnail size larger than the css size. A rescan didn't generate new ones so I deleted my database and rescanned. After rescan the thumbnails were crystal clear. You could try going into your server and deleting the thumbnail folder but it just error when loading the image, that or just rename the folder they are located in and if it doesn't generate you could reverse the name change.

To regenerate thumbnails without performing a full rescan, you need:

• to have the "Scan collection at launch" option checked (in the "General" settings tab)
• to change the size of the thumbnails in the settings and restart the server by using the blue button on the top right corner of the screen.

But thumbnail extraction and generation is what takes probably about 95% of the scanning duration. So you might as well  clear your database (there is also a button for that) and rescan everything.