Web filtering problem on one VLAN only

[CBInfo]

Hi,

I have multiple VLANs that are working just fine. The last one I created cannot acces some Microsoft website and the username field on iCloud.com won't show up for example. I tried to redo the VLAN multiple times the see where I f*d-up. I cloned the rules from another VLAN that is working fine. I created a rule allowing Microsoft365 Internet services and portal.office.com finaly loads but there is other glitches and you can see a lag when the page load.

Any help will be appreciated.

Thank you

Edit: even with web filtering disabled, iCloud won't work

Edit2: FortiGate 601E v7.2.8

Edit3: The problem is with the interface, not the VLAN

[CBInfo]

I did not change any policies, only the physical interface and it's now working. But of course I would like to know why the other interface is doing that.

[CBInfo]

In the CLI, I can see that there is a MTU override with a custom value on the problematic interface...