Rolling back to version 5.0.5 did it for mee also.
(luckily i did not have to re-create the forms and Keys)
– placed the 2 plugin folders of CF7 in plugins, renamed the one to version_505 before upload
– WP > plugins shows 2 contact form versions.
– deactivate 5.1 (no need to remove)
– activate 5.0.5
