This might be down to borderline reception of one frequency 11426 H. The Humax’s only look for this frequency on first install and use it to validate postcode. It comes from a slightly different satellite position from most together with a few other (non mainstream) channels. To prove this take one of the Humax’s to a friends and do the install. When you get back home it will function normally – unless you reset the unit.
So my guess is the dish is slightly west of 28.2E so that reception of 28.5E is marginal.