For some strange reason, Zwift stores its logs (and also a “TEMP” folder) in the Documents folder instead of the proper place on a Windows machine, which I believe is %appdata%. I, like millions of other people, keep my files synced with Microsoft OneDrive. Unfortunately OneDrive and Zwift fight with each other because Zwift is constantly writing to the logs (at least on startup) while at the same time OneDrive is trying to sync the file changes. This results in Zwift hanging on launch due to files being “in use” by OneDrive.
A “solution” was posted some time back (Move "working files" i.e. logs to Program Files (OneDrive issue)), which does not fix the issue. The suggested fix is to go into Settings > Account > Choose Folders and un-check the Logs folder. OneDrive warns you that un-checking a folder makes it unavailable on this PC, meaning the folder gets deleted (though the cloud copy remains). If you recreate the folder, OneDrive complains that there is now a sync issue because the same folder exists in the cloud. Deleting the cloud folder allows it to sync the Logs folder again, bringing you right back where you started.
Will Zwift fix their Windows app so that it stores logs and working temp files where they belong? That is the only real way to fix this issue.