Forum content reduced to third of screen size

[ex-EGLL]

Last night I logged onto PPrune, and the screen opened as normal (i.e. forum content occupied the full screen) After about a second the content only occupied the left third of the screen with a few adverts to the right of it and then the remaining half of the screen blank. The font size in the forum content didn't change, the boxes just became elongated to accommodate the text. See below.

Any ideas of cause and how to resolve?

https://cimg2.ibsrv.net/gimg/pprune.org-vbulletin/2000x1250/2023_02_03_8a1f410c941c6cee504fff22fffe6e7048a4c59a.png
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[ex-EGLL]

Tried connecting through a VPN but that made no difference. But if I open PPrune up in Edges's In Private or Incognito mode the page displays correctly. Clearing cookies and history didn't make any difference

ex-egll