Have you posted about this problem on the Formidable Community Forum or in the Formidable Masterminds Facebook group?
I am a long-time Formidable Forms user and have answered questions in Formidable public forums for over a decade. I have never heard of Formidable losing the capability to save entries because of a bug in Formidable itself without tracking it to some sort of a server-related issue like out dated PHP version or OS component(s).
There are recommended checklists you can run through if you’re a person that enjoys problem solving with a little support from a mentor. These checklists are provided by experienced developers who have been there and done that and enjoy teaching others how to navigate the intricacies of the WordPress/Formidable Forms environment. These are the developers that you’ll find volunteering their time in both of the groups I mention in the opening sentence.
Both of these groups welcome all Formidable Forms users, regardless of license tier, to the friendliest Formidable Forms support communities that you’ll find anywhere. I can safely say this because I am moderator on the Formidable Forms Community Forum and the owner of the Formidable Masterminds Facebook group. Please feel free to join either or both. There’s a very large community of Formidable Forms users eager to help!
Here are their links:
Formidable Forms Community: https://connect.formidableforms.com/
Formidable Masterminds Facebook group: https://www.facebook.com/groups/formidablemasterminds