Thanks Gerd,

My old ovm_* files were 236 MB. I havent created new img tiles yet with r3595.

If I run mkgmap on the old img files the same error is thrown out so I assume I must compile the tiles again.

I'll report it here later today because whole europe will take some time.

shapes turned out to be simple, with r3592 the overview map
file size is again smaller, esp. when the map contains
sea or forest polygons.
The ratio depends on the style, the 30% below were for the default
style which adds motorways.
For a map of Ireland, I see now 98KB with r3592 and 142KB with r3590,
both with "Openfietsmap full" style.

Gerd