@tekka Your rewrite actually fixed a problem I was having. For what ever reason I could not send messages from my ESP8266 based gateway to my test node. At the same time acknowledgement worked.
Log of the original lib:

0;255;3;0;9;send: 0-0-1-1 s=1,c=1,t=2,pt=4,l=4,sg=0,st=fail:1
0;255;3;0;9;send: 0-0-1-1 s=1,c=1,t=2,pt=4,l=4,sg=0,st=fail:1
0;255;3;0;9;send: 0-0-1-1 s=1,c=1,t=2,pt=4,l=4,sg=0,st=fail:1

Same sketch with just your commit added:

0;255;3;0;9;send: 0-0-1-1 s=1,c=1,t=2,pt=4,l=4,sg=0,st=ok:1
0;255;3;0;9;send: 0-0-1-1 s=1,c=1,t=2,pt=4,l=4,sg=0,st=ok:1
0;255;3;0;9;send: 0-0-1-1 s=1,c=1,t=2,pt=4,l=4,sg=0,st=ok:1

Nothing else was changed.
I didn't do any extensive testing. Just a quick check to see if your commit fixed my problem.

@tekka Your rewrite actually fixed a problem I was having. For what ever reason I could not send messages from my ESP8266 based gateway to my test node. At the same time acknowledgement worked.
Log of the original lib:

0;255;3;0;9;send: 0-0-1-1 s=1,c=1,t=2,pt=4,l=4,sg=0,st=fail:1
0;255;3;0;9;send: 0-0-1-1 s=1,c=1,t=2,pt=4,l=4,sg=0,st=fail:1
0;255;3;0;9;send: 0-0-1-1 s=1,c=1,t=2,pt=4,l=4,sg=0,st=fail:1

Same sketch with just your commit added:

0;255;3;0;9;send: 0-0-1-1 s=1,c=1,t=2,pt=4,l=4,sg=0,st=ok:1
0;255;3;0;9;send: 0-0-1-1 s=1,c=1,t=2,pt=4,l=4,sg=0,st=ok:1
0;255;3;0;9;send: 0-0-1-1 s=1,c=1,t=2,pt=4,l=4,sg=0,st=ok:1

Nothing else was changed.
I didn't do any extensive testing. Just a quick check to see if your commit fixed my problem.

Mosquitto uses an old version of the MQTT protocoll which is causing the error. So you need to tell the MQTT implementation to use the old version. I'm not sure if this is the intended way to fix it with MySensors, but for the used libary you have to go to drivers\pubsubclient\src\PubSubClient.h. Somewhere around line 20 there is a
//#define MQTT_VERSION MQTT_VERSION_3_1
#define MQTT_VERSION MQTT_VERSION_3_1_1
By swapping it to:
#define MQTT_VERSION MQTT_VERSION_3_1
//#define MQTT_VERSION MQTT_VERSION_3_1_1
you should be able to use your Mosquitto version.