Recently became the sharepoint admin where I work and had a user stumble upon this issue, and haven't been able to locate a possible "fix".
When the user attempted to insert a URL from a sharepoint site he did not have access to he received a 403 Forbidden error (as he should). However the problem came in that it defaulted to that URL every time he selected the "From Sharepoint" option and it immediately gave him the 403 error again with no way to change it.
The biggest problem here is that he isn't allowed to have access to that sharepoint site, so how can I change the default URL without granting him permission to that site so he can still insert links from sharepoint that he does have access to?