I just wanted to tell about how I could fix some severe problems with my Z5C and probably this solves also problems for others...
After I changed my service provider mid-June everything seemed to work fine (at least this was what I thought or I didn't pay too much attention or just didn't notice). But after nearly a month with the new carrier during my holidays, I got really annoyed by the phone and nearly had thrown it several times against a wall or so.
My experienced symptoms were:
1. Extreme lags throughout all possible areas of usage
2. Sometimes the phone stated that the SIM card was 'not available' and after a while it reconnected again (without restart of the phone). One incoming phone call was disconnected directly after taking it due to such a 'SIM loss'
3. The battery drain was higher than it used to be, main consumer 'Android System'
Initially, I didn't conect the problems to my carrier changing. So to solve the issues, I tried to do a system repair using Xperia Companion, but that didn't work and always aborted with errors. Also the system backup was not working (probably should have tried without the SIM once, comes to my mind now...). Often restarting the phone also didn't help, neither uninstalling or stopping unused apps.
What I finally made was to 'download internet and MMS settings' under Options-->More which updated the APN settings (sorry, my translation from German Android). And since then, my Z5C behaves as smooth as before :-)
How it was even working without these settings, I can only explain with the fact that my old and new carrier use the same infrastructure (Switzerland:
Swosscom --> Wingo). So my problem was probably very specific, but maybe there's still someone out there who can profit from my experience.
BTW 1, my wife also has a Z5C, also changed the carrier to the same one I did and also had such problems
BTW 2, my Z5C is still at 7.0, I haven't gotten the 7.1.1 update notice. My wife has, strange.