I submitted an issue on the BH1750 library, which was fixed. So OneTime modes now work as expected.
https://github.com/claws/BH1750/issues/17

Very pleased with my Elecrow order as well. This is the first pcb I designed since the 1980's when you taped up the art work by hand :-)

I wouldn't call downgrading a solution, but it is a useful workaround :-)

Since this isn't actually solved and because I want to not downgrade the sdk I did some tracing.
I added printf's to MySensorCore.cpp to see how far we get before the exception occurs. The ledsInit() at line 113 returns , but we crash somewhere during the hwReadConfigBlock() call to get the isMetric parameter. If I comment that out execution continues but fails after calling presentation().
This seems to implicate the EEPROM library calls from MyHwESP8266.cpp and indeed when I add printf's there (oh how I hate debugging this way!!!), it fails on the call to EEPROM.init(length).

The regulator has 3 terminals, on one side, 2 on the other. Usually near 2 'large' capacitors

@gerritv I will answer the question myself.

The directory layout of MySensors is a bit unusual, requires the following layout in platformio.ini

[platformio]
lib_dir = ../lib

[env:nanoatmega328]
platform = atmelavr
framework = arduino
board = nanoatmega328
build_flags = -I../lib/MySensors
lib_ignore = MySensors

The directory layout I use is:
Top level, a directory called lib with the contents of MySensors development branch. Inside this top level directory you have your various projects relating to this Solution:
A directory for your sensor node sketch, e.g, LivingRoomNode which contains the Platformio directory and the above info in its ini file
A directory for your gateway, with the same library parameters

The result is a solution with multiple source projects pointing at the same MySensors library.

Platformio is a great way to develop ESP8266/Arduino etc code. Also Platformio works very well with Visual Studio 2015 CE (this is what I build my MySensors nodes and gateways with).

Being Dutch I am very stubborn, a bit like a leach with lockjaw when it comes to defects.
@hek
Some more searching today turned up issues with esp8266/Arduino and the yield() function. That led to me looking through the MySensors site again and it turns out that a month ago in this set of comments that http://www.mysensors.org/build/esp8266_gateway user sl118 solved the problem by commenting out a line in MyGatewayTransportEthernet.cpp. I should have twigged on that earlier as someone elsewhere commented the using UDP solved the problem as well.

The setnodelay(true) enables the Nagle algorithm inside the tcp stack, this combines small packets into a larger one. So perhaps a bit of a performance hit on a busy network but at least I can use esp8266/Arduino 2.1 now and avoid using Arduino IDE for gw, and use VS2015CE for all my development again.

The credit should really go to @sle118 but I will create a pull request later today.

And now that we know exactly what to look for, I found this: https://github.com/esp8266/Arduino/issues/1695 where they library fix is being implemented as well as a description of the problem.