I had a similar issue and it was related to dnsfilter configuration.
I had the following error on Fortimanager:
data not exist
reloadconffail 'datasrc invalid. object: dnsfilter profile ftgd-dns filters category 22. detail: 90. solution: data not exist
You can find the details of the error on the task monitor.
Since we were not using dns filter, I have purged the configuration on the fortigate:
# config dnsfilter profile
Then the import on Fortimanager worked fine.