We have been experiencing some crashes when we view an Excel sheet in the viewer pane, and/or open an Excel sheet via Opus. Viewing PDF's are also often sluggish. I noticed the viewer pane states "(Adobe PDF for Vista (32-bit))" and we are using Windows 7 and have 64-bit pc's. I wouldn't think this would cause an issue, but would like you to confirm. What could be causing these problems?
Which process is crashing? Is it dopus.exe or something else, e.g. docsvw64.exe?
Excel and PDF viewers are not part of Opus; Opus merely hosts whichever viewers are installed on your system. The reported names also come from the viewers themselves, and are displayed so you can tell which components are being used.
Adobe have not updated the name of their viewer component in over a decade, as they are slack. 
Other PDF viewers may be faster for you.
Which viewer name is displayed when you view Excel files?
Leo, it's interesting to read your reply. So, after the filename is Microsoft Excel (32-bit). So my problem was that DOpus was freezing. I have it open now on the other monitor and after waiting a while, it is working. So I guess DOpus is using the Windows Excel viewer. It is also reading an ods file. So if I didn't have Excel installed, I guess Windows could view, but not edit Excel files. So, I think the problem is that the viewer might be slow to start and I should just wait.
Can you view Excel files in File Explorer? Do you get the same issues?
I don't know if this is relevant or not, but isn't that a file created by Open Office or Libre Office (or can Excel optionally create that format)?
This is interesting - I have kind of similar experiences, over long time.
Some times, but without notices or other specific situations to connect directly to the crashes; open excel files AND directory opus freezes when trying to open other excel files from the directory window.
I can not assure that this also have happened when I don't have the previewer window open - but I will observe this from now on.
It seems that this is more likely to happen when I have a number of excel files open - - more likely the more files open (6-8-10 files, individually not very big or complex).
Since this seems not to be only my specific configuration/windows related , it is interesting to know more about what causes the problem.
I have QuickView installed as file viewer, but it seems that also the "microsoft excel previewer" often is active (and reports an error: This file can not be previewed because of an error in the Excel Previewer" )
When killing the two dopus processes with task manager, the frozen excel file windows normally returns back to "live" status, and I can save the files normally. It is not enough (or even possible) to just close dopus by normal "file exit".
Does the same happen in File Explorer if you do the same steps there?