Reverting to the thumbnail column requires closing the tab and doing an "undo close tab" on it. Then the thumbnail column re-apppears and replaces the two symbol columns.
If you resize the column so it's larger, do you see thumbnails? (It will show icons when very small, and I think the width may not always be restored properly, although I'm still looking into that.)
I'm out of ideas on things to check, but if you want to send a config backup to me (leo@gpsoft.com.au) + steps on exactly what to click to trigger the problem, I'll see if I can reproduce it in a test VM.
Addendum:
This issue is also healed by ANY folder navigation.
If I start Dopus, the thumbnail column is broken in all tabs (like described above), additionally there's icons in front of the file name.
Then, if I open a folder or go up one folder level, and then back, everything is normal again: thumbnail column is there and working (in the respective tab), and no icons in front of the file name anymore.
Thanks for the config. With the help of that we now know how to reproduce the problem. (The default/custom format has to have the thumbnail column turned on, as well as the format saved in the layout which is then loaded.)
Possibly this still needs a little fixing occasionally, although I can live with it for the time being, apart from the opening of new tabs without my beloved thumbnail column due to this workaround.