Hello, @hkheidel
Welcome to the Canvas Community! Priscilla has given you some great links. I'm wondering if you might be experiencing an issue with an unsupported/out-of-date browser. Please try these troubleshooting steps:
(1) First, are you using Safari? Please check the version of Safari you're using (by clicking on Safari at the upper left and then About Safari) to ensure that you're using Safari 11 or 12. If you're not, you'll need to update the Safari browser to the latest version. (If you're not using Safari, skip to Step 3.)
(2) If you're already on the latest version of Safari, clear the cache and cookies in the Safari browser, close and then relaunch the browser.
(2) If these remedies do not work, try switching to a different browser. Canvas works quite well on a Mac through Chrome, and Firefox works well also. Here's a list of supported browsers: Which browsers does Canvas support?
If this doesn't resolve the issue for you, please contact Canvas Support. General instructions are in How do I get help with Canvas as a student? but your school's support setup might vary somewhat, in which case ask your local IT Department for additional troubleshooting assistance.