I don't know that the log files will be particularly helpful in troubleshooting the issue you describe, but in general, you will want to look at the Application and Script Log (located in ThingworxStorage/logs). These log files are accessible from within ThingWorx as well (see the Monitoring menu of Composer). In addition to our logs, most web browsers come with developer tools that you can use to troubleshoot Services and Mashups.
It may be best to open a formal support case for this issue. Are you seeing this behavior consistently? Have you tried clearing your browser's cache?
The only thing that springs in my mind when reading about your problem is that maybe you're pressing the "View Mashup" button exactly after you pressed the Save button, so that the Mashup is in a "Saving" state when trying to view it. To explain better, when you press the View button, the System first saves it, and then tries to open it. If it's currently in the process of saving it, then it will popup a very short message saying "Already saving" and it will not open it.
Other familiar problem that I see all the time, but I don't think it is related to your case, is that the Chrome browser blocks by default the new popup window which is spawned by the View Mashup button. See if there is a small message on the right side of the address bar when trying to open a mashup...but again, it might not be the case.
Hope it helps.
Thanks for your comments and tips.
Interestingly, when I first opened the composer this morning, I was able to use the view mashup button successfully on the first attempt, but not subsequent attempts.
I did find that there was a popup blocker in place so I've removed that and it seems to work consistently now. I really should have been able to work that one out myself.
The direct launch also works.
And now I know where to look for logging info.
So all in all, a useful set of replies - thanks again