DSO Nano V3 capturing single signal


I have a DSO Nano V3 (DS201+ Ossiloscope App ver.4.0).
It’s a very nice scope, but I can’t get the SINGL (single) syncMode to work correctly.
I want to look at serial communication (rs-232). If a communication pulse goes into the DSO Nano, I would like it to stay on the screen until a new pulse arrives. I think this is a job for the single syncMode.

My test setup is an arduino that sends out a quick pulse (+5Volts) on a button push.

In syncMode AUTO, the signal syncs nicely on the rising or falling edge of the pulse (the threshold is good), but in syncMode SINGL, nothing happens. I followed example 4 (Acquire Single Signal) of the DSO Nano V3 User’s manual, but without success.
When I go out of the trigger menu, the scope stops measuring, the syncMode name (SINGL) changes in red letter to STOP, and the trigMode (rising or falling edge) and the Sensitivity color red as well.
No signal is captured in this state.
When I click on hardware button ‘A’ (Play/Stop), everything becomes green again but also no signal is captured here.

What am I doing wrong?
I also looked at the BenF firmware, but I am confused if I should install it or not.

Serial: E1C19FB2 Licence: 23F4777C
Device Firmware Upgrade V3.22A

All the best!

I have a DSO Nano V3 app. 4.00 and the Single trigger doesn’t work also on my equipment. I think it’s a bug on the firmware and I hope will be solved as soon as possible because the equipment is not conform to the specs. I wait a week after I return it to the seller.

Is it a solution to upgrade to the Benf-firmware?

As reported above, I’m confused if this is safe or not with version 3