I submitted this problem as a bug on Microsoft Connect (https://connect.microsoft.com/VisualStudio/feedback/details/1074514/error-list-has-disappeared). The ensuing response solved the issue:
Looking at the data you've provided, I believe you may have hit a
known issue with the new Error List in Visual Studio 2015 Preview,
which manifests itself when you attempt to double-click at the edge of
a column to resize. The resize works, but the next time you restart
Visual Studio the error list fails to load.
If you see this problem again, it’s possible to work around it to get
your error list back up and running again. This requires editing the
registry (which should always be undertaken with caution, after
backing up the registry)
The relevant key is
HKEY_CURRENT_USER\Software\Microsoft\VisualStudio\14.0\NewTaskList
Follow these steps:
- Close all running copies of Visual Studio on the affected machine
- Open RegEdit and locate the relevant key
- Delete the whole key (it will be regenerated when Visual Studio 2015 Preview re-starts)
- Re-start Visual Studio 2015, and the Error List should be available again
We have fixed the underlying bug in later builds - watch for new
releases of Visual Studio 2015. Thanks for taking the time to try
Visual Studio 2015 Preview