Nice tune, Louise!
A couple of points...
In the header, before the tune coding starts properly, the final field should be the Key field in this case K:G. Nothing else should come after that, only the tune code. ABC syntax requires this, although it is possible to get away without it depending on your ABC software. The initial time signature M:3/4 should come before the K: field.
It is also good ABC practice to embed further time signature changes in square brackets. This allows you to close up the lines of coding and remove the \ line breaks.
As it stands your coding works OK with Barfly for Mac, but I think the following coding is better:
X:1
T:Hobla mr d'Spae
C:Zwiefacher Trad. aus Wurttemberg,Germany
L:1/8
M:3/4
K:G
"G"G2>A2 Bc | B2>B2 AB |[M:2/4] "D" c2A2 | "G"d4 |[M:3/4] G2>A2 Bc | B4d2 |[M:2/4] "D"A2B2 | "G"G4 :|
[M:3/4] "G"Bdd2B2 | "D"Acc2A2 |[M:2/4] "G"G2B2 |"D"A4 |[M:3/4] "G"Bdd2B2 | "C"c2c2ce|[M:2/4] "D"d2F2 | "G"G4:|
I also had no problem converting it to midi format using Barfly. But haven't tried the back conversion. It's not something Barfly does, nor I use.