Seems that the lockable knobs seizing due to the mild steel fittings and the outside living is a common issue! (Mild steel rod and nut)I've had a Thule Wanderway 2 bike rack fitted on my T6 for the last 18 months. I use it several times a month for MTB/gravel events so I leave it fitted all the time, plus I don't have anywhere to store it when not on the van!
It has been brilliant.... however I've had two issues with them recently;
1. Both the locking clamps that connect the bike to the rack have seized up. Once I raised this with the retailer I bought it from they told me it was my fault as I shouldn't leave them outside all the time!! Quite a ridiculous response given it's for use outside and there is no recommendation from Thule about this. Initially they told me I'd need to buy new ones for well over £100, but Thule have a no quibble 2 year warranty so they have both now been fixed FOC. I will now be keeping the clamps inside until I need to use them - worth considering if you do the same as me and were leaving them on all the time!.
All three of mine seized solid so i stripped them down and replaced the mild steel with stainless steel. A fully useable solution for £30