SP 365 rest search api return '500 internal server error' for using HTTP POST - sharepoint-rest-api

Question

Our product uses SP 365 search api. Several of our customers report that when using the following search API: '/_api/search/postquery' with body:
{'request': {
'Querytext':'test',
'SourceId':'8413cd39-2156-4e00-b54d-11efd9abdb89',
'RowLimit':400,
'SelectProperties':{
'results':['Title','Path','Description','Write','Rank','Size']},
'TrimDuplicates':true,
'ClientType':'Custom',
'Culture':1030,
'SortList':{'results':[{'Property':'Rank','Direction':'1'}]}
}}
returns HTTP status code 500 with error text (inside json response): 'An unknown error occurred.'
However, if one adds to the query above a condition to limit it to a specific SP list, such as:
{'request': {
'Querytext':'test AND "ListId":{A7B96B28-6062-435B-A2EE-4792512A95A1}',
'SourceId':'8413cd39-2156-4e00-b54d-11efd9abdb89',
'RowLimit':400,
'SelectProperties':{
'results':['Title','Path','Description','Write','Rank','Size']},
'TrimDuplicates':true,
'ClientType':'Custom',
'Culture':1030,
'SortList':{'results':[{'Property':'Rank','Direction':'1'}]}
}}
then the query works well.
This happens for specific tenants like 'pfgroupas.sharepoint.com'.
This API calls have been working for years now (started with SP 2013) & are broken in the last few days.

Answer 1

Turns out that, for certain tenants, the internal server error manifests itself if you specify the 'SortList' parameter. If you create a query without it, like:
{'request': {
'Querytext':'test',
'SourceId':'8413cd39-2156-4e00-b54d-11efd9abdb89',
'RowLimit':400,
'SelectProperties':{
'results':['Title','Path','Description','Write','Rank','Size']},
'TrimDuplicates':true,
'ClientType':'Custom',
'Culture':1030
}}
then everything works fine. Seems to me like a server side bug.