Sync Gateway errors - Couldn't serialize JSON and 500 Internal error: read tcp xxx.xx.xx.xxx:4985->xxx.xx.xxx.xx:56178: i/o timeout

I am analyzing Sync Gateway logs and found following lines:

2020-10-30T07:38:42.293+03:00 HTTP: #4962: --> 500 Internal error: read tcp 172.17.215.118:4985->172.17.222.94:56178: i/o timeout (87993.0 ms)
2020-10-30T07:38:42.295+03:00 HTTP: #4963: --> 500 Internal error: read tcp 172.17.215.118:4985->172.17.222.94:56179: i/o timeout (87993.1 ms)
2020-10-30T07:38:42.296+03:00 HTTP: #4964: --> 500 Internal error: read tcp 172.17.215.118:4985-

And also these lines:

2020-10-30T07:37:30.286+03:00 WARNING: Couldn’t serialize JSON for &{495b14e2-4139-4cec-abb9-7c5895baf607 495b14e2-4139-4cec-abb9-7c5895baf607 0xc4222cb560 map[ContactPhone:89122423362 profileType:4 type:PledgeObjectTaskContact _id:495b14e2-4139-4cec-abb9-7c5895baf607 _rev:1-a7a9cf47d7eafe0dd66a3fb6980db10b ContactEmail: ContactPerson:Тестовое контактное лицо] 0 0} : write tcp 172.17.215.118:4985->172.17.222.94:52667: i/o timeout – rest.(*handler).addJSON() at handler.go:621
2020-10-30T07:37:30.286+03:00 WARNING: Couldn’t serialize JSON for &{fe685b6a-c492-4dd2-a470-84c87e91d17b fe685b6a-c492-4dd2-a470-84c87e91d17b 0xc4222cbbc0 map[ObjectType:1 taskId:97b95884-c0f4-4a34-a144-4db5b0d8b0e2 _id:fe685b6a-c492-4dd2-a470-84c87e91d17b _rev:1-998e6f7f4ddc65365ba6fc81b5ce50c8 AllowSync:true RevisionId:22 isAccepted:false profileType:4 type:QuestionnaireResult LastRevisionDate:1603858504814 employeeId:upper_admin_1 isApprovedByUser:false inspectorId:aspect_inspector_4 questionnaireSchema:0ba7dcc4-f4d9-4c47-91f1-df2e553440a0] 0 0} : write tcp 172.17.215.118:4985->172.17.222.94:52667: i/o timeout – rest.(*handler).addJSON() at handler.go:621

What are the reasons for this case?

P.S.: I am using Sync Gateway 2.0

I got the same error messages sometimes after migrating to Couchbase 6.6 and SyncGateway 2.8 (lastes version).

Any reason why I see these messages?