Please follow the troubleshooting tips below:
– deactivate all plugins to see if this resolves the problem. If this works, re-activate the plugins one by one until you find the problematic plugin(s).
– switch to the Twenty Twenty theme to rule out any theme-specific problems.
– reset the plugins folder by FTP or PhpMyAdmin. Sometimes, an apparently inactive plugin can still cause problems.
– re-install WordPress. Sometimes, due to incomplete FTP transfers, your install might not work properly. Head over to Dashboard > Updates > click the Re-Install Now button.
The steps above are part of the basic WordPress troubleshooting process and are all reversible. Let me know if this helps.