HTC One M8 Lollipop Problems

You need to do a factory reset on the phone.  You have an app that did not like the conversion to lollipop.  Back up all your data and factory reset the phone.  

Then clear your cache.

To wipe the cache partition, follow these steps:

If the phone is not working correctly, you may need to master reset it and also clear the cache partition. These two resets clear different parts of the phone storage. Unlike a master reset, wiping the cache partition does not delete your personal data.

To wipe the cache partition, follow these steps:

1. Turn off the device.
2. Press and hold the Volume Down key.
3. Press and release the Power key to turn the phone on.
4. Continue holding the Volume Down.
5. Release the Volume Down key when the three Android images appear on the bottom of the screen.
6. Press the Volume Down key once to move the cursor from FASTBOOT to RECOVERY.

1. Press the Power key to begin the wipe.
2. The device displays the bootup screen followed by a phone with a red triangle.
   
3. Press the Up Volume key and Power key at the same time.
4. Press the Down Volume key three times to select Wipe cache partition.

1. Press the Power key to initiate the wipe.
2. Wait until the phone displays Cache wipe complete.
3. Press the Power key to select Reboot system now.
4. If this does not resolve your issue, the next step a master reset