Hello LukaszRz,
I think validation will always fail because there has been a change in internal file system starting 17 release.
An upgrade with "no-validate" is more likely to be successful.
If you are not comfortable with "no-validate" , I would suggest you to get JTAC involved when you are upgrading.
Also, as per the upgrade guidelines you are encouraged to upgrades "NOT more than 3 releases" at a time.
Refer :- https://www.juniper.net/documentation/en_US/junos/information-products/topic-collections/release-notes/18.2/jd0e27340.html#cbbu-rn-junos-es-upgrade-downgrade
"You can upgrade or downgrade to the EEOL release that occurs directly before or after the currently installed EEOL release, or to two EEOL releases before or after. For example, Junos OS Releases 15.1X49, 17.3, 17.4, 18.1 and 18.2 are EEOL releases. You can upgrade from one Junos OS Release to the next release or one release after the next release. For example you can upgrade from Junos OS Release 15.1X49 to Release 17.3 or 17.4, Junos OS Release 17.4 to Release 18.1 or 18.2, and from Junos OS Release 18.1 to Release 18.2 or 18.3 and so on."
Hope this is helpful.
Thanks!