If you start annotating a File Share and the slide is replaced by a blank canvas, check whether the shared document's filename contains a "+" symbol. This is a known issue and a fix is planned for a future release. Removing the plus sign should resolve the problem; if it doesn't, try removing any other special characters, though the plus sign is the only one currently confirmed to cause this behavior.
Below is a brief animation showcasing the blank canvas issue caused by the unsupported file name character:
If you are in the middle of a live class session and encounter this issue, you would need to stop the File Share activity, then start a new one with a file that has a corrected name.