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Oh, you had solved this already. Posted a comment without refreshing the discussion first. Sorry and thank you a lo!
The problem was disappeared today (Wed 25.5.2015). One sensor name includes scands so the error came from somewhere else. Please inform if the problem was generated due to misuse of Tellmon. Thank you.
Hi,
The username is vesa.auervirta@gmail.com.

Normally I use the Tellmon app "Sensors" this way:

First I unselect the sensor offered by the app by default: Sensor "1. Ulkolämpötila (temp)" and "1. Ulkolämpötila (humidity)".

Next I select sensor "4. AT Tuloilma kenno (temp)" and "4. AT Tuloilma kenno (humidity)".
The error appears right after I click the selection square in front of "4. AT Tuloilma kenno (temp).

I have renamed sensor names about the time I started with Tellmon, but Tellmon has worked fine until these days as theError 400 appeared.

Would it be worth making a small change in a sensor name causing the error, or do you want to trace this error before any changes? Both ways are fine for me. I am not in a hurry with this matter.