Have a general question on Licence LPGL 2.1
Is it free of charge if we provide Xwiki installed on our cloud platform for commercial customers?
Yes it is. But if you make some changes on the core sources of XWiki, your customers must be able to get them.
To be precise you must provide XWiki and the changes you bring to it under the same license (ie LGPL 2.1) to your customers. See GNU Lesser General Public License v2.1 (LGPL-2.1) Explained in Plain English - TLDRLegal for details.