I've seen plenty of gibberish on Navtext, especially Nasa receivers, but I have never seen those non-characters that you have on the 2nd line up. I wonder if the firmware on your device has got screwed.
Navtex is in the MF spectrum where thousands of household devices nearby cause plentiful interference. It works much, much better out at sea. The protocol (SITOR-B) is also fairly ancient and has poor error correction. That said, if you set the filters for sea area and message type (tables and sea area map can be found somewhere near the front of your Reeds Almanac), you will end up with fewer garbled messages that don't concern you anyways. I have the same model and in general it works quite well, but it is perfectly normal to see the odd gibberish while in a marina.
If after setting it up correctly you only ever see garbled messages, even when out at sea, it may be knackered - the pixel blocks that don't look like characters could mean that. NASA has good tech support from what I hear (never needed it yet myself) - give that a try then.