You can find the following entry in the ULS-Log:

Solutions2Share.Solutions.CollaborationManager.EventReceivers.WorkspaceListEventReceiver.ItemAdding: Item does not exist. It may have been deleted by another user.  ExceptionType: 'ArgumentException'  ExceptionMessage: 'Item does not exist. It may have been deleted by another user.'  StackTrace: ' 

at Microsoft.SharePoint.SPList.GetItemById(String strId, Int32 id, String strRootFolder, Boolean cacheRowsetAndId, String strViewFields, Boolean bDatesInUtc, Boolean bExpandQuery)     

at Solutions2Share.Solutions.CollaborationManager.Services.Functions.GetTemplateByWorkspaceField(TemplateLookup templateField, SPSite site, Boolean sync)     

at Solutions2Share.Solutions.CollaborationManager.EventReceivers.WorkspaceListEventReceiver.ItemAdding(SPItemEventProperties properties)'  Source: 'Microsoft.SharePoint'  TargetSite: 'Microsoft.SharePoint.SPListItem GetItemById(System.String, Int32, System.String, Boolean, System.String, Boolean, Boolean)'  

Solution:

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Please navigate to the Template Library and add the Users / Group to the library permissions with Read Permission Level