Thanks for reaching out to us. We are very pleased to support you.
It is recommended that you try the following to troubleshoot the problem and resolve it:
- Are all of your external users experiencing the same issue when using Share Connect? Please try sending the share link for other external users to see if the problem persists. If only some of your users are affected, then I suggest you remove the external users and re-add them to your tenant. Please contact your administrator to go to M365 admin Center > Active User to re-add the external user.
- If all external users are affected, then it is likely that the “Control access to apps that don't use modern authentication” setting in Office 365 is disabled.
- Please contact your SharePoint administrator to go to the SharePoint Admin Center.
- In the left-hand menu, select Policies.
- Under Policies, select Access control.
- Look for the option labeled Apps that don’t use modern authentication. Toggle the setting to Allow access.
If you have any questions, please do not hesitate to contact me.
Moreover, if the issue can be fixed successfully, please click "Accept Answer" so that we can better archive the case and the other community members who are suffering the same issue can benefit from it.
Your kind contribution is much appreciated.