Try it without the columns environment in your document, i.e.

\startchapter[title=\mytitle{Register}]
\placeregister[index]
\stopchapter


Hans van der Meer schrieb am 26.08.18 um 17:48:
> Could the following setup I used be the culprit?
>
> \setupregister[index][pagestyle=slanted,textstyle=small,alternative=A]
>
>
>> On 26 Aug 2018, at 17:43, Wolfgang Schuster 
>> <wolfgang.schuster.lists@gmail.com 
>> <mailto:wolfgang.schuster.lists@gmail.com>> wrote:
>>
>> The real question is why do you get an index without two columns.
>>
>> \starttext
>>
>> \dorecurse{26}{\convertnumber{a}{#1}\expanded{\index{\convertnumber{a}{#1}}}}
>>
>> \placeregister[index]%[n=3]
>>
>> \stoptext
>>
>>
>>
>> Hans van der Meer schrieb am 26.08.18 um 17:37:
>>> How do I produce an index with more than one column?
>>>
>>> This produces one column only:
>>> \startchapter[title=\mytitle{Register}]
>>> \startcolumns[n=2]
>>> \placeregister[index]
>>> \stopcolumns
>>>
>>> Nor did \placeregister[index][n=2] or \placeindex[n=2], with or 
>>> without the columns envirenment, produce the result wanted.
>>> The \startsimplecolumns[n=2] environment did produce two columns, 
>>> but then played havoc with the textheight.
>>>
>>> dr. Hans van der Meer
>>>
>>>
>>>
>>>
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>>
>
> met vriendelijke groet
> dr. Hans van der Meer
>
>