I tried again with your setup, my file was also ca. 55 mb. Jessie image with configured LMS to Stretch image with freshly installed LMS. The Jessie version had an older install of LMS 7.9.0 while the Stretch had LMS 7.9.1.
Unfortunately, I could not reproduce your error. With LMS running, I again got:
Trying to Restore Squeezebox Server Settings…
File move successful
Settings successfully restored! Please restart Squeezebox Server now.
Please share your debug info from the Squeezebox Server menu.
Since I have not heard back from cramcram, I assume he is yet to try using a normal browser instead of Samba. Please let me know if you can reproduce Mike’s error.
If so, please share your debug info log.