Since the 2017.08 update, I have not been able to mix down to a wav file in Sonar Platinum.  The export does result in a file, but the file is one I am not familiar with.  Under type, the file is listed as "5 file". The size of the file seems consistent with what I would expect, but I have no idea what a "5 file" is or how to derive the audio from it. 

it it possible the file is a wave file but with the extension .5 instead of .wav? Are file name extensions displayed in Explorer?

I'm running: 23.8.0. BUILD 30 [2017.08] - x64
 
Seems to be running fine for me (wav.file) I have a new song individual tracks, then mixed down to stereo.  

Just pay attention to the dialog boxes when you open the export. It's possible you hit something that chose this. 
See the last entry I made bold, this might indicate you have chosen a surround format of some kind. 
 
From Sonars Documentation "Export Dialog" 
Files of type: 
Select from one of the following export formats:

Wav. The standard digital audio format used under Windows and for CD’s, with a file extension of .wav.

Broadcast Wave (time-stamped). A wave file with the following embedded information:
Description. A brief description of the contents of the Broadcast wave. Limited to 256 characters.
Originator. The author of the Broadcast wave. This information is taken from the Author field in the File Info dialog.
Originator Reference. A unique reference identifier created by SONAR.
Origination Date. The date the file was created.
Origination Time. The time the file was created.
Time Reference. The SMPTE time stamp for the beginning of broadcast wave.

Windows Media Advanced Streaming Format. The Microsoft Corporation's compressed audio format, with the file extension of .wma. If you choose Windows Media Advanced Streaming Format you will be prompted for more information about the file by the Microsoft Audio Encode Options dialog box.

MP3. The mpeg-3 format, with the file extension .mp3.

SoundCloud – CD Quality. Share a CD quality version of your song on SoundCloud. This format will result in a larger file size and may take longer to upload.

SoundCloud – FLAC. Share a FLAC version of your song on SoundCloud. FLAC is a royalty free high quality lossless audio compression codec. Files compressed with FLAC can be 30-50% smaller than the original wav file and unlike MP3 there is no loss of audio quality. Use this preset if you want to have a share a higher quality version of your song, but also want to have a shorter upload time to SoundCloud. Your song will be saved as a FLAC (.flac) file.

SoundCloud – MP3. Share an MP3 version of your song on SoundCloud. Your song will be saved as an MP3 (.mp3) file. MP3 files are the smallest and fastest files to upload to SoundCloud, however they will not be as high quality as Wave or FLAC files.

Various surround formats. If you have installed any 3rd-party surround encoders, you will see them listed here.

"The 5 file extension is associated with IBM Embedded ViaVoice, is a range of language-specific continuous speech recognition software products developed by IBM. The current version is designed primarily for use in embedded devices.
The 5 files are used for language specific script data."
 
Don't know what this means but this is what Google has to say about it.

scook, no it's listed as "5 file". I went to file.org. and this what it informed me:
 
"While, we do not yet have a description of the 5 file format and what it is normally used for, we do know which programs are known to open these files.":
                                   winrar
                                   dx designer
                                   pro/engineer
                                   creo elements/pro
                                   3d-tool
                                  

Ok, rscain, another one to add to the list.

Cactus Music.......I'll double check again.

Is it possible your project name has a ".5" in it?

 Well I've double checked and tried it several times.  I export a project to wave and instead, I get a 5 file.