Dear Murdaich.
Thank you for you answer.
But in the meantime I found out that my hub (which was quite new) was not working properly. Bought a new one. Puter immediately communicated with the printer. Whew.
Start page problem: I power up and get a phantom paper jam. Power off and on again, tries to print a startup page but gives a real paper jam. Pull toner cassette out, remove jammed paper, put toner back in, close lid. No more paper jam LED.
Go to the page I want to print, give print command and set paper feed to multipurpose. If paper is of the right thickness (thin, like 80 grams per square meter) and the humidity is ok, the printer works well.
Also, on Ebay I bought another 12/640 PS for EUR 19.-- transport from Paris to here is EUR 87.70. Not too bad I think.
Thank you again for your help !
Takeo.