FortiGate Next Generation Firewall utilizes purpose-built security processors and threat intelligence security services from FortiGuard labs to deliver top-rated protection and high performance, including encrypted traffic.
Even when following the recommended upgrade path, some settings may be lost after the upgrade due to a difference in supported features between the firmware versions.

The following command can be used in order to list configuration errors resulted during the upgrade process and boot up.
# diagnose debug config-error-log read

An example of a commonly reported error is as follows:

>>> "unset" "post-lang" @ root.firewall.profile-protocol-options.scan.ftp:command parse error (error -61)

The line above indicates an issue with the parameter "post-lang", inside the FTP configuration of a firewall Proxy Options profile called "scan".  Here is the CLI configuration section:
config firewall profile-protocol-options
edit scan
config ftp
unset post-lang

The next suitable step would be to confirm in the CLI Guide of the particular FortiOS firmware version whether the parameter 'post-lang' still exists, was removed, or renamed.