This issue was introduced by a new version of the macOS OneDrive client. OneDrive overrides the default application setting for all online-only files so that those files are always opened by the "OneDrive File Launcher" app instead of the default application (e.g. Word for .docx files). When opening an online-only file in Finder, the "OneDrive File Launcher" app performs the file download and then opens the downloaded file in the default application. This fails for file paths on the Boxcryptor drive which are not recognized by OneDrive.
We plan to release a new Boxcryptor for macOS next week which will contain a fix and restore the default application for online-only files in the Boxcryptor drive. Until then, a workaround is to use "Open With" (Right-click an online-only file in Finder -> Open With) to manually select the default application instead of the "OneDrive File Launcher" app.