i have the same problem, on precheck 1 host whith esxi 6.0.0 was 5.5.
i cant delete host from inventory because my veeam backup will nonconsistent.
so, lets start
1)connect to vc over ssh
2)use postgresql to edit vcdb
/opt/vmware/vpostgres/9.4/bin/psql -d VCDB postgres
3)check table for incorrect entry
SELECT ip_address, product_version, product_build FROM vpx_host;
4) if one of your host have incorrect esxi version in db just change it))))) [change version, build and ip to your data]
UPDATE vpx_host SET product_version = '6.0.0' WHERE ip_address = '10.0.0.1';
UPDATE vpx_host SET product_build = '7967664' WHERE ip_address = '10.0.0.1';
5)run again precheck and it will work fine)))