Try fixes for the Forza 5 disconnected error here:
https://www.gamerevolution.com/guides/697160-fo...
https://www.gosunoob.com/guides/forza-horizon-5...
https://www.reddit.com/r/forza/comments/qqml3a/...
https://www.reddit.com/r/ForzaHorizon/comments/...
https://www.reddit.com/r/forza/comments/qnxx78/...
https://attackofthefanboy.com/guides/forza-hori...
https://quoramarketing.com/fix-forza-horizon-5-...
https://respawnagain.org/forza-horizon-5-discon...
https://www.youtube.com/watch?v=lL9a1b-nXM4
Feel free to ask back any questions. Based on the results you post back I may have other suggestions if necessary.
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