See https://legacy.curseforge.com/wow/addons/worldbosstimers?comment=367

In short the (saved?) position of the GUI window gets corrupted (nil?).

Needs more investigation. Suggested causes from the thread:

• Enabling account wide position when the hidden
• In use together with Altoholic

Toggling the "Account-wide GUI position" option while the GUI is hidden will trigger this bug. The result is that the GUI position object will still exist, but some of its fields will be removed.

WBT won't even initialize correctly after this happens since this is persistent data, so the problem can't be fixed except by restoring default settings. I'll add a check which detects if the position is corrupted and then fallback to default, to repair for anyone affected by this.