The Humax PVR-150T will definitely have problems using an indoor set top aerial; this is true of almost all Freeview receivers. Freeview signals are typically hundreds of times weaker than terrestrial transmissions and if you've managed with a set top aerial so far it's been working more by luck than design. I find that most set top aerials are so badly made that they're often faulty when new, but as most tend to flogged cheaply at markets and car boot sales the supply chain has no incentive to improve the product.
There has recently been a national Freeview retune, to complete this correctly your box needs the kind of solid and reliable signal which you cannot get with a set top aerial. Some receivers had compatibility issues with the retune and in many cases manufacturers broadcast OAD's (Over Air Downloads) to update the firmware in the boxes to be compatible. OAD's almost never successfully install via set top aerials.
There is an official list of Freeview devices which are known to have problems with the retune
; unfortunately it has not proven to be a comprehensive list. The Humax MG-TU1 is one of those which have had problems and from memory it's closely related to your Humax PVR. The official advice is that you contact Humax directly on 0870 247 8800, but they'll probably instruct you to test your box using the aerial system it's designed to use before they'll be interested in offering further assistance.