Currently, the App Inventor server is experiencing high usage and longer than normal build times.
I expect Projects with large media files will have issues loading. Try an experiment; make a copy of a Project; remove some of the media files and see what happens when you try to load.
If your class is using a school network with an on site classroom, the issue could be your school network sharing bandwidth with many individuals and other classes and perhaps Zoom; if these are students at home, the issue may be problems with their Internet providers due to high usage.
Does the problem occur when you load a simple Project (place a Button on a Designer, then use Companion to run it or build it). What happens. Is it loading while these other Projects are not?
Make sure your students are using the latest Companion 2.60. Using an 'old' Companion could cause issues.
Regards,
Steve