One of the cases I've seen was due to a disabled account. the following was logged in the most recent Trisoft.InfoShare.Web*.log file :
ExternalPreview:Guest /ISHCM/Api/ExternalPreview Trisoft.InfoShare.Web.Modules.InfoShareSessionAuthenticationModule.ConvertClaimsPrincipal []
Trisoft.InfoShare.Core.AccessDeniedUserDisabledException: Your account has been disabled.
The ISHCM web.config file (\InfoShare\Web\Author\ASP\Web.config) has a section
trisoft.infoshare.web.externalpreviewmodule that contains the external ID of the account that is used to log in on behalf of a anonymous user. (The ExternalPreview works without authentication aka anonymous but underneath a log in into Tridion Docs is happening.)
Check if the user is still enabled. Perhaps try to log in as this user to see if this still works and if other error messages are shown.