A few times, probably after using Link2SD to move an app from the SD card to the phone internal storage or convert a user app into a system app, I received the following message

Although it looks scary, it is easy to fix. The key to resolve the error is to look on your device in:

/data/system/uiderrors.txt

If you have rooted your device, which if you've been using something like Link2SD is most likely, use something like ES File explorer, making sure that you've enabled root access and open the above file, scroll to the bottom of the log to find and delete the APK which is causing the error. Otherwise, read on!

Here is an example of mine where the package "com.wordreference" was the cause of the problem.

1. Connect your device to your computer, fire up ADB and get a shell session
   
   wisedom@wisedom-thinkpad ~ $ adb devices

   * daemon not running. starting it now on port 5037 *

   * daemon started successfully *

   List of devices attached

   01547AE81900F006 device
   

   wisedom@wisedom-thinkpad ~ $ adb shell

   
   Above, I've checked that my device (in bold) was connected before firing up a shell session.
   
   

2. Display the content of /data/system/uiderrors.txt root@android:/ # cat /data/system/uiderrors.txt    

   08/04/2015 18:13: No settings file; creating initial state

   4/8/15 8:20 PM: Need to read from backup settings file

   4/8/15 9:21 PM: System package com.google.android.ears no longer exists; wiping its data

   ….

   ….                                                                                                                        

   29/06/2015 07:04: System package com.vp.alarmClockPlusDock no longer exists; wiping its data 02/07/2015 20:10: Need to read from backup settings file

   06/07/2015 12:18: Need to read from backup settings file                                

   09/07/2015 21:31: Need to read from backup settings file                                

   09/07/2015 21:31: Package com.wordreference has mismatched uid: 10085 on disk, 10127 in settings

   10/07/2015 00:14: Need to read from backup stopped packages file

   Above (in bold), you can clearly see which package was responsible for the error.
   
   

3. Next, all needed to be done was to delete the package (I had 2 my in case). root@android:/ # rm /data/app/com.wordreference-1.apk root@android:/ # rm /data/app/com.wordreference-2.apk
4. Reboot your device to finish.