Actually I found something in conversion log by manually "grep'ing" the file. Seems they are not available in the troubleshooter page log files.
03:14:57.431 [domain1.com][Error] Message: Critical error loading domain (domain1.com) after conversion
Exception: Utf8Json.JsonParsingException: expected:'{', actual:' ', at offset:0
at Utf8Json.JsonReader.ReadIsBeginObjectWithVerify()
at Utf8Json.Formatters.SmarterMail_Common_Files_DomainFiles_DomainAccountsFileFormatter32.Deserialize(JsonReader& , IJsonFormatterResolver )
at Utf8Json.JsonSerializer.Deserialize[T](Byte[] bytes, Int32 offset, IJsonFormatterResolver resolver)
at SmarterMail.Common.Files.FileFunctions.ParseJsonFile[T](Byte[] data)
at SmarterMail.Common.Files.FileFunctions.ReadJsonFile[T](String path, String cacheBusterKey, Action onLoadFromDisk)
at SmarterMail.Common.Files.DomainFiles.DomainAccountsFile.Read(BlockIdGenFile genFile, String path, String cacheBusterKey)
at MailService.Repositories.DomainRepository.ReloadUserRepositories()
at MailService.Repositories.DomainRepository.LoadRequiredItemsFromRepository()
03:14:58.294 [domain2.com][Error] Message: Critical error loading domain (domain2.com) after conversion
Exception: Utf8Json.JsonParsingException: expected:'{', actual:' ', at offset:0
at Utf8Json.JsonReader.ReadIsBeginObjectWithVerify()
at Utf8Json.Formatters.SmarterMail_Common_Files_DomainFiles_DomainAccountsFileFormatter32.Deserialize(JsonReader& , IJsonFormatterResolver )
at Utf8Json.JsonSerializer.Deserialize[T](Byte[] bytes, Int32 offset, IJsonFormatterResolver resolver)
at SmarterMail.Common.Files.FileFunctions.ParseJsonFile[T](Byte[] data)
at SmarterMail.Common.Files.FileFunctions.ReadJsonFile[T](String path, String cacheBusterKey, Action onLoadFromDisk)
at SmarterMail.Common.Files.DomainFiles.DomainAccountsFile.Read(BlockIdGenFile genFile, String path, String cacheBusterKey)
at MailService.Repositories.DomainRepository.ReloadUserRepositories()
at MailService.Repositories.DomainRepository.LoadRequiredItemsFromRepository()
03:15:08.747 [domain3.com][Error] Message: Critical error loading domain (domain3.com) after conversion
Exception: Utf8Json.JsonParsingException: expected:'{', actual:' ', at offset:0
at Utf8Json.JsonReader.ReadIsBeginObjectWithVerify()
at Utf8Json.Formatters.SmarterMail_Common_Files_DomainFiles_DomainAccountsFileFormatter32.Deserialize(JsonReader& , IJsonFormatterResolver )
at Utf8Json.JsonSerializer.Deserialize[T](Byte[] bytes, Int32 offset, IJsonFormatterResolver resolver)
at SmarterMail.Common.Files.FileFunctions.ParseJsonFile[T](Byte[] data)
at SmarterMail.Common.Files.FileFunctions.ReadJsonFile[T](String path, String cacheBusterKey, Action onLoadFromDisk)
at SmarterMail.Common.Files.DomainFiles.DomainAccountsFile.Read(BlockIdGenFile genFile, String path, String cacheBusterKey)
at MailService.Repositories.DomainRepository.ReloadUserRepositories()
at MailService.Repositories.DomainRepository.LoadRequiredItemsFromRepository()
03:15:10.919 [domain4.com][Error] Message: Critical error loading domain (domain4.com) after conversion
Exception: Utf8Json.JsonParsingException: expected:'{', actual:' ', at offset:0
at Utf8Json.JsonReader.ReadIsBeginObjectWithVerify()
at Utf8Json.Formatters.SmarterMail_Common_Files_DomainFiles_DomainAccountsFileFormatter32.Deserialize(JsonReader& , IJsonFormatterResolver )
at Utf8Json.JsonSerializer.Deserialize[T](Byte[] bytes, Int32 offset, IJsonFormatterResolver resolver)
at SmarterMail.Common.Files.FileFunctions.ParseJsonFile[T](Byte[] data)
at SmarterMail.Common.Files.FileFunctions.ReadJsonFile[T](String path, String cacheBusterKey, Action onLoadFromDisk)
at SmarterMail.Common.Files.DomainFiles.DomainAccountsFile.Read(BlockIdGenFile genFile, String path, String cacheBusterKey)
at MailService.Repositories.DomainRepository.ReloadUserRepositories()
at MailService.Repositories.DomainRepository.LoadRequiredItemsFromRepository()
Real domain names redacted for privacy...