Luca, sometimes ago i had a problem with vlan fail safe, I opened a case on F5 support. and the Engineer said:
" One of the contributing factors to the issue you're experiencing is from your failover settings. Currently unit 1 is set to prefer active, and unit 2 is set to prefer standby. This will cause unit 1 to fight for active status anytime there is no failover condition present. I recommend you set both units to have no failover preference.
Also, a note about VLAN failsafe: this feature does not just monitor the connection between the F5 and the switch, but the entire VLAN. If the F5 looses connection to that VLAN do to an issue elsewhere in the network, neither unit will be able to be active and they will ping pong. Unless there is a specific reason you need to monitor for VLAN communicate I would strongly recommend you remove VLAN failsafe from your configuration. This feature is only useful in very select circumstances and usually causes failover problems as you've been experiencing "