I get the following message when I submit my form:

Warning: [ err 2/line 391/config.cls.php] session_start(): open(/opt/alt/php83/var/lib/php/session/sess_3rur0vguq94u4eb1svef4esec4, O_RDWR) failed: No such file or directory (2)
Warning: [ err 2/line 391/config.cls.php] session_start(): Failed to read session data: files (path: /opt/alt/php83/var/lib/php/session)
Warning: [ err 2/line 391/config.cls.php] session_start(): open(/opt/alt/php83/var/lib/php/session/sess_3rur0vguq94u4eb1svef4esec4, O_RDWR) failed: No such file or directory (2)
Warning: [ err 2/line 391/config.cls.php] session_start(): Failed to read session data: files (path: /opt/alt/php83/var/lib/php/session)

Any ideas what is going on?

What version and build number for Web Form Builder are you using??

Have you manually exported the form and placed the appropriate files in the correct place as per the instructions??