Hello,
Checkout the "Fixing broken dials page" article I've wrote recently.
https://github.com/fastaddons/GroupSpeedDial/wiki/Fixing-broken-dials-page

Sadly, database implementation in all major browsers is fragile so sometimes thumbnails breaks. It happens to some users more often, for others it never happens. It's hard to say why, could be also sign of SSD issues.

If you need further assistance, please write me email or create new issue on my github page.

Best regards,
Juraj Mäsiar