Either firmware or wiring. Or just temperatures 
@foosel Octoprint ist der Hammer! Sehr geil gemacht! Vielen Dank.
Octoprint is a great product.
I am coming also from a development background and I am sorry to say it but your are only partly right. I went through the errors and normal terminal messages in the last days. OctoPrint is just not able to display temp with less then 10°. I thing the label and the Temp chart are expecting a three digit temp. (XX.X and not X.X)
Update from today surrounding temp 3° when connecting to the printer:
Terminal giving at the end this Error:
Connected to: Serial<id=0x6d144490, open=True>(port='/dev/ttyUSB0', baudrate=115200, bytesize=8, parity='N', stopbits=1, timeout=10.0, xonxoff=False, rtscts=False, dsrdtr=False), starting monitorStarting baud rate detection...Changing monitoring state from "Opening serial port" to "Detecting baudrate"Trying baudrate: 115200Send: N0 M110 N0125Recv: startChanging monitoring state from "Detecting baudrate" to "Operational"Recv: echo: External ResetRecv: MarlinSend: N0 M110 N0125Recv: echo: Last Updated: 2015-12-01 12:00 | Author: (CR-10 500)Recv: Compiled: Jun 1 2018Recv: echo: Free Memory: 1139 PlannerBufferBytes: 1232Recv: echo:Hardcoded Default Settings LoadedRecv: echo:Steps per unit:Recv: echo: M92 X80.00 Y80.00 Z400.00 E93.00Recv: echo:Maximum feedrates (mm/s):Recv: echo: M203 X300.00 Y300.00 Z5.00 E25.00Recv: echo:Maximum Acceleration (mm/s2):Recv: echo: M201 X300 Y300 Z100 E5000Recv: echo:Accelerations: P=printing, R=retract and T=travelRecv: echo: M204 P500.00 R500.00 T1000.00Recv: echo:Advanced variables: S=Min feedrate (mm/s), T=Min travel feedrate (mm/s), B=minimum segment time (ms), X=maximum XY jerk (mm/s), Z=maximum Z jerk (mm/s), E=maximum E jerk (mm/s)Recv: echo: M205 S0.00 T0.00 B20000 X5.00 Z0.40 E5.00Recv: echo:Home offset (mm):Recv: echo: M206 X0.00 Y0.00 Z0.00Recv: echo:Material heatup parameters:Recv: echo: M145 M0 H185 B0 F0Recv: echo: M145 M1 H240 B0 F0Recv: echo:PID settings:Recv: echo: M301 P22.20 I1.08 D114.00 C100.00 L20Recv: echo:Filament settings: DisabledRecv: echo: M200 D3.00Recv: echo: M200 D0Recv: echo:SD init failRecv: echo:SD init failRecv: okSend: N1 M115*39Recv: FIRMWARE_NAME:Marlin 1.1.0 From Archive SOURCE_CODE_URL:http:// ... PROTOCOL_VERSION:1.0 MACHINE_TYPE:www.cxsw3d.com EXTRUDER_COUNT:1 UUID:00000000-0000-0000-0000-000000000000Recv: okSend: M21Recv: echo:SD init failRecv: okSend: M105Recv: ok T:6.9 /0.0 B:6.4 /0.0 T0:6.9 /0.0 @:0 B@:0Send: M105Recv: ok T:8.4 /0.0 B:6.1 /0.0 T0:8.4 /0.0 @:0 B@:0Recv: Error:MINTEMP triggered, system stopped! Heater_ID: bedSend: M105Recv: ok T:6.6 /0.0 B:5.6 /0.0 T0:6.6 /0.0 @:0 B@:0Recv: Error:MINTEMP triggered, system stopped! Heater_ID: bedSend: M105Recv: ok T:8.4 /0.0 B:6.4 /0.0 T0:8.4 /0.0 @:0 B@:0Send: M105Recv: ok T:8.1 /0.0 B:6.4 /0.0 T0:8.1 /0.0 @:0 B@:0
I could solve the issue with the octoprint setting in the second state at "Verhalten" were it is not killing the connection to the print. It would only to a current print. So I am able to set the temp and warm up the bed and the nozzle above the Mintemp (which must be between something like 5° to 9°)
So the Error might be triggered from the firmware but is neg liable.
But the temp chart and the labels of octoprint are not displaying temp under 10° even there is no Error. This would be nice if it could be changed, but as I now it from my clients,
I think am a very rare case so thank you all for your support. It help me a lot under standing the messages and how to find a suitable solution 
It is able to just fine. But the temperature tab simply considers anything less than 10° as "off" for display purposes. Seemed more practical that way.
Doesn't change though that that Error: is triggered in the firmware, not OctoPrint. OctoPrint is just displaying it. Note how it says "Recv" = "Received" in front. Your firmware decides that less than 10°C is not a viable temperature for your printer (or that your thermistor is broken), triggers that error and locks itself down. This is not OctoPrint doing that, it's your printer.
edit actual temperatures below 10°C will be displayed with their value instead of off starting with 1.3.11 to prevent anyone from thinking that the temperature graph and the firmware triggering mintemp are related
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