Actually, I had this problem same as you. You can get the "Forbidden" error if your commit includes different directories ; Like external items.
And i solved in one step. Just commit external items in another case.
Additionally, I advise you to read articles on External Items in Subversion and VisualSVN Server:
VisualSVN Team's article about Daily Use Guide External Items. It explains the principles of External Items in SVN.
https://tortoisesvn.net/docs/release/TortoiseSVN_en/tsvn-dug-externals.html