Search REST API – An unknown error occurred

Wondering if you could help me please, I am currently calling the SharePoint rest search api with a number of parameters as below:

GET: https://tenant.sharepoint.com/sites/sitecol/_api/search/query? querytext='path:https://tenant.sharepoint.com/sites/sitecol ContentTypeId:0x0101008CA34DF8F4904B3EB1EB49AC10614C68*' &trimduplicates=false &startrow=0 &rowlimit=1 &selectproperties='Write,Path,LastModifiedTime'&refiners='LastModifiedTime(discretize=manual/2017-12-01/2018-01-01/2018-03-01/2018-03-01/2018-04-01/2018-05-01/2018-06-01/2018-07-01/2018-08-01/2018-09-01/2018-10-01/2018-11-01/2018-12-01/2019-01-01/2019-03-01/2019-03-01/2019-04-01/2019-05-01/2019-06-01/2019-07-01,sort=name/descending,filter=99/1/*)' &ClientType='ContentSearchRegular' 

I am using the header Accept: application/json;odata=nometadata

However when running this in Postman or SharePoint online I get the following returned:

{ “odata.error”: { “code”: “-1, Microsoft.Office.Server.Search.REST.SearchServiceException”, “message”: { “lang”: “en-US”, “value”: “An unknown error occurred.” } } }

The only method I appear to have found to fix this is to reduce the number of thresholds in the refiner parameter however I am unable to find any documentation detailing the maximum number of thresholds that can be used.

I appreciate any help that anyone can give on this matter.