Is it possible to somehow "force" DOpus to use the existing thumbnails cache ?
The situation is: a large cache has already been generated, but, all of a sudden, DOpus is re-generating the thumbs in all folders... I checked in the default location for the cache, and all the files are still there, but DOpus is completely ignoring those I tried a system restore, restoring old DOpus settings, re-copying the *.db files... to no avail thus far.
I suspect that something changed in the video codec configuration (ffdshow) that made DOpus decide to "refresh" all thumbs...
DOpus will only regenerate the cache if certain thumbnail settings in the Opus Preferences have changed, or the source images have changed (timestamp or size). When either of these things happen there's no way to force it to use the old cache, unfortunately.
The settings in thumbnail Preferences that are important are thumbnail size and the state of the 'high quality scaling' flag - if any of these have changed you could try changing them back, which should make Opus accept the old cache files again. If the file timestamps or sizes have changed there's not much you can do unless you can restore these as well.
The thing is, I double-checked and I'm pretty sure none of those changed. Neither the settings you mention, nor the file sizes or timestamps (certainly not in all folders). Furthermore, it seems that the video codec settings have nothing to do with re-generating thumbnails.
Then, I really don't understand what happened... And it's actually not the first time :-/
Just a heads-up. The mysterious loss of link to an existing thumbnails cache could well be due to... the automatic time changing in Windows for "daylight savings"...
Apparently, when Win (XP SP3) adjusts the time by itself, Dopus seems to think the image timestamps have changed, and thus entirely "ignores" all existing thumbs.
From a quick (not definitive) look at the code, Opus seems to use UTC timestamps when checking if the thumbnail cache is valid, so it should not be affected by daylight savings (unless I have missed something; it was only a very quick look).
What type/format of drive are the files in question on? If it's not an NTFS drive then the issue may be that the filesystem/OS itself reports shifted times, since some filesystems (e.g. FAT) store timestamps in local time. [1]
I tried a system restore, restoring old DOpus settings, re-copying the *.db files... to no avail thus far.