Unspellable,
I have this happen too often because I write too much.
It has nothing to do with your cookies or whether you do or do not a have the "remember me" box checked. Nor does it have anything to do with your ISP. It's this site's time delay software settings. They exist to avoid clogging the site with people who log in and go out for lunch, overloading the site.
But this can be overdone, and the pause time setting for this site's software is far too short, in my humble opinion. This should be easily fixable, but that feature may not be accessible or visible to our host, depending upon how the software was written. Maybe the software designer or seller would know. But it's not worth worrying about or bothering the site owner, because the user work-around is easy.
When you get the Dread Message with the empty white boxes, if you simply single click the "Login" button, you should move along normally and everything you drafted should appear in preview,
if you had just clicked the preview button before the dread message appeared, that is. If you click the other button (not "Login"), the whole works is lost. You start from scratch. And don't fill in the empty white boxes. You'll just foul up the works.
So the way to dodge this whole mess:
1. If you draft a long message, never just click "Submit Reply"when ready to do so. Click "Preview" first, mostly to find out whether you have waited too long.
2. If you then get the Dread Message, click only the "Login" button. You will see your message previewed and you can move along normally; post it or continue to edit.
3. Then, don't wait as long before clicking "Submit Reply". ...or you're back at Square One.
Doing it as above, there's no need to draft within one application and then cut and paste into another. Simpler is always better.