Radio module used in FX30 is WP8548/WP7601/WP7603. Though FX30 has WP8548 module there will be difference in the firmwares, You can’t use the firmware of WP8548 on FX30.
You need to use the FX30 (Ethernet) 3G firmware whcih is in the this link source.sierra.
I went through logs attached, I couldn’t find what is the reason. Using firmware of WP8548 on FX30 might be the reason for this SOTA issue, But not sure. Please try with the official FX30 firmware which i have mentioned above.
You were able to perform SOTA updates successfully using previous legato versions? OR you are facing issue only with Legato-17.19.0. I would suggest you to use Legato-16.10.1 as this is legato version which comes with latest official FX30 release.