New fields failed to get created properly, resulting in DB errors
As Demerit mentioned, you have the same issues I reported. I don't think it's about the number of fields that's the issue, but the combination of data types. If you are receiving that error, be careful if your site is live because some of your cases will hit an error and you won't be able to view the case page.

Here's what I did:

If possible, you will want to delete all the custom fields and the custom group itself. Do it in phpmyadmin because it's faster. Once you have cleared the data, split the 65 fields into two custom groups. The error should go away. 

If you can't delete the custom group because you already have data, delete the custom fields and then create a new custom group to split off some of those fields.