External Sharing not working with Communication sites

I tried do enable external sharing. 

Enabeling the feature worked, but accepting the invitation fails.

Set-PnPTenantSite -Url https://xxxx.sharepoint.com/sites/test-aaa -Sharing ExternalUserSharingOnly

(Get-SPOSite -Identity https://xxxx.sharepoint.com/sites/TEST-aaa).SharingCapability
ExternalUserSharingOnly

(I tried Set-PnPTenantSite -Url https://xxxx.sharepoint.com/sites/test-aaa -Sharing ExternalUserAndGuestSharing too and ith worked too, but the outcome was the same):

The invitation is always sent to the external user. But when accepting the invitation sent, the external user always get a error (sorry for German but have not found a way to trigger this error in English):

Das hat leider nicht geklappt.
Leider können wir die Einladung für "x.y@outlook.com" im Moment nicht annehmen. Versuchen Sie es bitte später erneut. In der Zwischenzeit versuchen wir, das Problem automatisch zu beheben.
Wenn das nicht weiterführt, wenden Sie sich an die Person, die Sie eingeladen hat, und geben Sie diese technischen Details an:
Korrelations-ID: cc0a1e9e-e082---
Datum und Uhrzeit: 29.09.2017 09:37:25
URL: https://xxxx.sharepoint.com/sites/test-aaa/_layouts/15/acceptinvite.aspx?invitation={6FD65184-----E56687AC9DAF}&signInProvider=live
Benutzer: x.y@outlook.com
Problemtyp: Fehler beim Akzeptieren der Einladung

Last line reads "Issue Type: Failed to accept invitation."

I checked this https://support.office.com/en-us/article/Turn-external-sharing-on-or-off-for-SharePoint-Online-6288296a-b6b7-4ea4-b4ed-c297bf833e30#ID0EAABAAA=Office_365_Groups and http://www.techmikael.com/2017/08/enable-external-sharing-on.html I also followed the conversation here: https://techcommunity.microsoft.com/t5/SharePoint/Anonymous-Sharing-not-working-with-Communication-sites/td-p/84966

When I look at Site Settings -> Access requests and invitations I find the invitation under "External user invitations". Before and after I clicked on the invitation link.

I tried several external mail-addresses and several browsers (Chrome, IE and Firefox)

Everythings seems Ok. But I can't find the error.  I would be very pleased about tips and hints.