@Marcus Apologies if this has been mentioned, but are you using the sweetened (i.e. Vanilla) versions of Huel or using the flavour pouches? Quite a few people (including myself) are sensitive to artificial sweeteners including Sucralose. Typically one or two Huels containing Sucralose is OK for me, but if I consume any more than that, in additional to other artificially sweetened drinks (e.g. sweetener in coffee or in diet coke), then I start to get headaches.
Sucralose in particular is known for triggering migraines in people that are sensitive to them.