There is an old issue with hidden tabs, which is still exisitng in the new 15.1 release.
You will find the older issues by searching for "focus hidden".
Error: _"Can't move focus to the control because it is invisible, not enabled, or of a type that does not accept the focus."_
The problem is limited to IE8 and doesn't occur on Firefox, IE10 or Chrome. I don't know if you still support IE8, but a fix will be easy if you do so.
AjaxControlToolkit\AjaxControlToolkit\Scripts\Tabs.js (line 695)
```
_setFocus: function(obj) {
$get("__tab_" + obj.get_element().id).focus();
},
```
A simple if-statement should fix the problem.
Comments: The issue was fixed, and the fix is included in v15.1.3
You will find the older issues by searching for "focus hidden".
Error: _"Can't move focus to the control because it is invisible, not enabled, or of a type that does not accept the focus."_
The problem is limited to IE8 and doesn't occur on Firefox, IE10 or Chrome. I don't know if you still support IE8, but a fix will be easy if you do so.
AjaxControlToolkit\AjaxControlToolkit\Scripts\Tabs.js (line 695)
```
_setFocus: function(obj) {
$get("__tab_" + obj.get_element().id).focus();
},
```
A simple if-statement should fix the problem.
Comments: The issue was fixed, and the fix is included in v15.1.3