I never had a real problem getting the Android versions of TT to run on various devices - until I updated my Nexus7 (gen2) to Kitkat.
Since I fixed it I thought I put all possible causes and fixes in one place.
All does not mean all! Only the problems that I'm aware of and was able to fix, so don't blame it on me if you still can't get it to work

One of the most common cuases of problems for noobs is simply a wrong installation.
(I assume you got the right APK and map as well as the correct activation if needed)
Might sound hard, but if you already tried without luck it is best to fully remove Tomtom before proceeding!
To make sure you got the folders right I recommend to install the APK only and to start it.
Tomtom will create a folder called "tomtom", within there the "com.tomtom.COUNTRY-NAME..." folder an all content (later on).
If all goes well you should see this Tomtom folder on your SD - if not, don't panic!
Your Tomtom will complain about being unable to connect to TT - that is normal as we have not added any content yet.
On many devices there will be no Tomtom folder being created on the SD,
so copy the "Tomtom" folder from your download (or that you created for a non-Android map) to your SD and start TT again.
In most cases the app will find the content folder on the SD and it just works.
If not:

The missing dot...
I noticed in many cases that the .metadata.xml file does not have the leadin dot, so it is only metadata.xml - TT will not work without the dot in front, so if missing simply change the filemane
Ok, still not working?? Keep reading...

Sometimes it happens that either you or someone else messed up the activation.
Possible causes are:
The map was activated with the deviceID of a different APK than what you use - if unsure simply create a new DCT inside the map folder with the correct activation.
You activated again with correct values but you did not delete the old DCT file first - some keygens don't overwrite existing DCT files! So if in doubt delete or rename the DCT file before you activate again.

Last but not least....
Of course it can also happen that for some reason you downloaded an unpatched APK from dubious sources (that we don't support anyway) or that you try to use a map that is not compatible.
In general all full featured maps for the GO series and of course the Android maps can be used, not other map versions are support by the Android TT version.

Hard core....
In my special case all of the above was ruled out and I even tried the content folder from a working installation on a different device.
Modifying the .metadata.xml like suggested in other threads (like setting the size to zero or adjusting the path) only resulted in two things:
a) "unable to connect" or
b) the app closed with a sorry note.
I knew the apk is good, same for the content on the emulated SD, so what was wrong?
Seemed Kitkat works a bit different or my Nexus just has a special tick:
Checking the "data/data" folder in the root I noticed the same folder structure that was on the SD only there was no content.
Copying all from the SD into the data folder worked but was not a solution I liked.
After a few experiments I noticed that only the .metadata.xml needed to be copied from the SD to the same location inside the data folder in the root to make TT working.
Although I alread got feedback that other users dont have such problems on Kitkat I though I share my findings anyway as I was able to replicate it 3 times after a full factory reset.
On the other hand, after flashing the Cyanogenmod onto the Nexus TT installed as usual with no fuzz at all.

I hope this might help someone in case nothing else seems to work.