Langen FIS frequency of 120.65 is close enough to be a culprit if it were used in the aircraft near the time of the GPS loss. You can easily test Com radio interference.
Other causes can be a bad GPS antenna on the aircraft, check for water leaks and or over torquing of the antenna causing cracks.
The VHF interference check is done easily enough by setting the GPS on a page which shows the space vehicles and their signal strengths. Usually it looks something like this
Then set the frequency – 121.15 is a nice one, 1575/13 – and press PTT for say 20 secs and see if the bars drop.
There are related factors e.g. the ELT PI-tank circuit getting excited by VHF TX near 121.50, and generating so much RF that it saturates the GPS input stage.
If it is comm interference these work very well in my experience. My GTX335 GPS would drop out immediately (in less than a second) when pushing the PTT on certain frequencies, but the issue was completely resolved for $130 USD plus a few minutes work. I was later told by a local installer that he uses them on every certified GPS installation to prevent similar problems.
Yes; exactly what I used to fix the KLN94 issue in the link above. I got well ripped off on the shipping costs though; the company doesn’t know / doesn’t want to know how to send a package to a “foreign country” so they use a 3rd party shipping house which charges c. $250 even for a tiny package 
For non-certified, you can just make your own stub filter for about £5 in materials.
Other causes can be a bad GPS antenna on the aircraft, check for water leaks and or over torquing of the antenna causing cracks.
That‘s true, I had it years ago, but the GPS of the iPad is not affected by this.
I learned to always put the ILS approach in standby. Helped me last time I accidentally put the GPS into some mode where it didn’t capture anything. The fact that something like this has already happened in the relatively short time I am flying IFR shows me how good that advice was :)
One needs to be a bit careful with that because some autopilots will not intercept a GS except
So if you have to make a rapid switch to the ILS, you may well have to hand fly it all the way down, otherwise you may get the [airbus joke] “WTF is it doing now” moment
Personally I would always fly an ILS even if I had LPV, because while the tracking (with a modern GPS navigator, and roll steering to the autopilot) will be much better – laterally – with LPV, the signal processing is much more robust on an ILS.
