FAQ
On a fresh install (i.e. not updating an existing release) the magisk app does not show up first. A reboot is required to make it available.
After the reboot you will find the Magisk app in your app drawer but it does not have a logo and when starting asks for downloading Magisk files.
This is expected (and wanted) behavior. See "After I hidden the Magisk app, the app icon is broken" for details (it applies to this situation as well).
There was a workaround possible before but the app itself can't be integrated as a system app anymore (magisk contains a stupid check for that) and HAS TO be installed in /data - which is not available during the ROM build.
also check the next FAQ
previously tracked here.
After a fresh install (and also after an OTA including an updated magisk version) the startup of magisk shows an error:
Simply select OK here and let it reboot once. In some cases this message will re-appear somewhen which you can simply ignore then.
The reason for this is similar to the previous FAQ: during build time there is no way to identify a valid partition to place magisk files. Even if we use (e.g.) the fstab found it would become complicated to pre-init this for magisk. It is possible, yes but not worth it (imho) as this is a one-time-and-forget task.
If you did the above and the message keeps appearing do not worry. While it is unknown atm why it does not have any negative impact when you simply ignore it from now on.