After working fine for a few days, the board is now slow again. It seems to happening only between (roughly) 03.00 and 07.00 GMT.
I guess during these times the server load or available bandwidth is reduced deliberatly for backups or something or that it is under heavy load or something like that.
Anyway, it again takes a long time to load a page (20 seconds or more sometimes) and occasionally I get "server not found". Every other page I visit during this period is working fine.