LMS scan does not detect MusicIP mixable tracks

I've been running LMS+MusicIP for years on Debian Wheezy, but decided a few days ago that it was time to upgrade to Stretch. As my set up has been stable for a very long time, I'm a bit rusty at configuring LMS and MIP to work together, but I think I've done it right. MIP is running before LMS starts. MIP shows the correct number of mixable tracks (so it sees its database at home/[username]/.MusicMagic/default.m3lib); and I have the MusicIP pluggin enabled on LMS. But after a 'Clear library and rescan everything', there are no items in the track context menus for creating mixes.

I've been trawling through searches for a couple of days now, and I'm completely stumped. I'd be very grateful for any advice about how to fix this.

Info:
Logitech Media Server Version: 7.9.1 - 1523349116
Perl Version: 5.24.1 - i686-linux-gnu-thread-multi-64int
Database Version: DBD::SQLite 1.34_01 (sqlite 3.7.7.1)

Thanks in advance for any help you can offer.