Forticlient IPSEC VPN and MTU troubles under linux

I am testing the IPSEC tunnel with the ztna client (unfortunately there is no forticlient vpn under linux that supports IPSEC), and had trouble with https sites, so I want to add some info for other victims.

Somehow my ethernet interface picked a MTU of 1280 instead of the 1500 default for ethernet. This caused that the MTU for the sites behind the VPN was 1170, so I could do a telnet to check that the port was open but couldn't open any site.

Forcing the MTU on the ethernet to 1500 fixed the issue and now I can browse all the sites without issue.