All links between the top 6 green tasks have been created from left to right and top to bottom.
No problem so far…
But if I remove the 1st green link on top left and try to create it again then checkCircularDependency raises an error.
I don’t understand as I don’t see any circular reference in this chart.

Any explaination is welcome.

it’s hard to tell without seeing the whole data (hierarchy in lefthand grid) and not being able to test gantt in browser. Could you export problematic data to json, so it could be loaded in another gantt and tested locally?

There are two cases when the error may appear - when there is an actual circle in dependences, or when you try creating a relation from parent to child task. It may be something related, although I can’t tell from the screenshot

I am having the same problem (dhtmlxgantt Enterprise version 4.0.0).

The following can be used to reproduce the error.
I am unable to create a link from Task 1.1 to Task 2.1 in the UI.

var tasks =  {
                {id:1, text:"Main Project", type:"project", order:1, open: true},
                {id:2, text:"Sub-Project 1", type:"project", order:2, open: true, parent:1},
                {id:3, text:"Task #1.1", start_date:"10-12-2015", duration:2, order:3, progress:0.6, parent: 2},
                {id:4, text:"Task #1.2", start_date:"12-12-2015", duration:2, order:4, progress:0.6, parent: 2},
                {id:5, text:"Task #1.3", start_date:"14-12-2015", duration:2, order:5, progress:0.6, parent: 2},
                {id:6, text:"Sub-Project 2", type:"project", order:2, open: true, parent:1},
                {id:7, text:"Task #2.1", start_date:"11-12-2015", duration:2, order:3, progress:0.6, parent: 6},
                {id:8, text:"Task #2.2", start_date:"13-12-2015", duration:2, order:4, progress:0.6, parent: 6},
                {id:9, text:"Task #2.3", start_date:"15-12-2015", duration:1, order:5, progress:0.6, parent: 6},
                {id:10, text:"Task #3", start_date:"17-12-2015", duration:2, order:5, progress:0.6, parent: 1},
            { id:1, source:3, target:4, type:"0"},
            { id:2, source:4, target:5, type:"0"},
            { id:3, source:5, target:10, type:"0"},
            { id:4, source:7, target:8, type:"0"},
            { id:5, source:8, target:9, type:"0"},
            { id:6, source:9, target:10, type:"0"},

gantt.config.auto_scheduling_initial = true;
gantt.config.auto_scheduling_strict = true;
gantt.config.auto_scheduling = true;


The test for circular dependency is failing within an inner loop of gantt._checkCircularDependency (in dhtmlxgantt_auto_scheduling.js), where it recurses into checking the links of the target task.
It sees that Task #3 was seen in the checks for links from Task 1.1. However this is not a circular dependency.

Hope that helps in tracking this issue down.