I had that problem on LG V30. I reverted to 16.0 with Manager 5.9.1, and the problem persisted. I then downgraded Manager to 5.8.3, and it's working fine again with the older version of Manager.
I had the problem after updating from 16.0 with 5.8.3 to 17.1 with 5.9.1, but I also tried using the newest manager with 16.0, and that didn't work either. So there must be some kind of issue with the newest version(s) of the manager. I completely uninstalled 17.1, flashed 16.0, updated manager to 5.9.1 (crashing returned), then uninstalled 5.9.1 and installed the 5.8.3 apk without changing Magisk. If you have 17.1 installed now, then you might want to try just replacing manager with an older version before you bother with completely uninstalling/downgrading Magisk. You can simply uninstall the manager app without uninstalling Magisk, but I don't know if Magisk 17 will work okay with the older versions of manager, since I haven't tried that myself and don't really know anything about Magisk.