NMU alerts Mobile Data usage is very high

Hi, I've been running the paid version of Norton Mobile Security (curr.: V3.11.0.2625), Norton Halt, Norton Snap [curr.: Ver. 1.0.1.62) as well as Norton Mobile Utilities (curr.: Ver. 2.7.1.362) since I bought my Sony Xperia Z in march 2013 without any problem until I upgraded to Android 5.0.2 Lollipop at the end of May 2015. From then on Norton Mobile Utilities keeps warning me every day that my usage of mobile data is way to high and increasing, which is very annoying. August has the record so far with a usage of ~ 95 GB(!) According to NMU I've used up 19.4 GB at the moment in September. According to the "mobile-data-usage-meter" in my phone, however, it is only ~810 MB. The subscription limit is 3 GB/month and my telephone- and internet- service provider has not complained yet. According to the bills my usage will hardly exceed 3 GB a month. So there seems to be an issue with NMU... Any idea how to solve the problem?