If it were to happen to me, my preferred approach would be:
Step 1: Clear data of Magisk Manager app
Whatever way your Android supports, open the App info page for your Magisk manager app, force stop the app, and clear its data/storage. Reboot (optional). Now launch the Magisk Manager app. If the problem still persists, follow the steps below.
Step 2: Uninstall Magisk (the boot image modifications) and Magisk Manager (the app)
You can do so from Magisk Manager app by chosing "Uninstall Magisk" at its home page/launching page. While you're at it, also remove the Magisk files and modules saved under
/data/adb. I don't know whether they would be automatically removed by Magisk Manager app upon uninstallation, so it doesn't hurt to purge the directory by yourself.
If the uninstall option is not available in Magisk Manager app, than manually restore the stock (unpatched, that is) boot image to your currently active boot slot. You can restore the stock boot image using a custom recovery, or using fastboot, or if
su (root access) is still working, than using
dd. If you don't have a custom recovery, you may find this Q&A helpful: How do you root a device with Magisk when it doesn't have a custom recovery
Whatever means you choose, do remove the Magisk Manager app beforehand. It would not make a difference if you do it later, by why wait to do it?
Note: if you had installed Magisk using a custom recovery earlier or had upgraded Magisk from Magisk Manager app, you would be having a backup of stock boot image under
/data/ with directory name(s) starting as
magisk_backup_ followed by some random characters. If you fail to find that backup, than search the internet to get a legitimate full OTA / fastboot image for your currently installed Android build. The boot image there is your stock boot image and can be flashed instead.
Step 3: Patch the boot image with Magisk modifications
Assuming that you have successfully restored the stock boot image and rebooted the device to a running Android, you need to install Magisk again. Magisk is a work in progress, and its preferred method of installation may change in future, so I suggest you follow its FAQ for installation.
If you have followed the installation steps, you should have a working Magisk modified boot image and a companion Magisk Manager app. Try launching the app to see if everything is working as you would want it to be.
At this point you should be able to install/sideload a Magisk module through Magisk Manager app.