Booting in Insecure mode

Hi,

I had to reinstall Ubuntu and since then my XPS 9370 always says 'Booting in insecure mode' even though Secure Boot is enabled in UEFI BIOS. Does this have anything to do with with Expert Key Management's keys not being correct? And if so, how do I add or generate or whatever new keys?  And what key (there's PK, KEK, db, dbx) ?

Thanks in advance for helping me out !