Seems the issue is present on all browser that navigate previous on the forum so if i solve on the PC 1 with browser clear the PC 2 still will show the error.
The solution is to SSH php flarum cache:clear
so maybe this should be done by Flarum by default when this is necessary. No sense i cannot edit settings or extension in the community and all time login to SSH for clear cache. I read exist a plugin but seems to be not compatible yet with beta 8.
Will be nice if the beta 8 can solve this issue.