Hi @LeslieM and welcome.
This thread appears to discuss a similar issue with some potential workarounds: Divi Builder does no longer work with CP - #14 by 1stepforward
ClassicPress started from WordPress 4.9.x as a base, and we haven’t made any changes to the functions or hooks that plugins use internally. This means “an incompatibility with core” is unlikely to be the right answer.
I would recommend that you confirm the same issue still affects WordPress 4.9.x (we can provide steps to move your site back to this version if you need them) and then take it up with the Divi support team. This will take ClassicPress out of the equation and make it easier for their team to find the issue.
Other people who use Divi will be able to help more, but I’m not sure how much we can do about vendors who claim to support the version of WordPress that ClassicPress is based on, but then release code that doesn’t live up to this claim.