OK, my strong suspicion here is Gandi.net does not meet the requirements for host data for javascript access, which is described here:
https://github.com/igvteam/igv.js/wiki/Data-Server-Requirements. In particular, given what you've told me, I am guessing the server is not including CORS headers. The instructions linked above describes setting these headers for Amazon AWS. Many hosting services, such as Dropbox and Google set these for you automatically, but they have to be configured for AWS, and potentially for Gandi. So I think that's where the issue is, you can investigate if Gandi has options for CORS (Cross Origin Resource Sharing), if they do not there is no way around this problem. It does not affect IGV desktop because CORS is enforced by the Browser. It also does not affect direct access in the browser address bar, CORS only applies to scripted access (i.e. javascript => igv web).