I have the same problem  with my ts401  after the update to 5.57  it wont power on.   so I did hold down n and power button  it would not turn on.   I open the unit up  unplugged the battery  for 10 seconds  still would not power on.  I tried many times unplugging the battery and plugging the battery back in   holding down the n button and  enter button  the unit powerd up.  ok great.   2 days later  same thing  unit wont power on.     so I got rid of the 5.57  went back to 5.56. works perfect now.   something is wrong with the 5.57 version.  I am waiting  for the 5.58 version  to come out   hold that fixes  the power on problem.

 I wonder if the memory flash size of the units aren't big enough anymore to accommodate anything larger and 5.56 might be the last one that will work?

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 Same problem.  I updated to 5.57 used the TS401 several times......then all of a sudden nothing. It would not power up.  Tried holding the [N] button & [power] button at the same time for 10 seconds.  It worked & powered on.  Updated again to 5.57 and is now working........maybe I’ll update to 5.58.