"Seams" in first layer My first layers started to have these "seams" in them.
I am printing on the  Anycubic Ultrabase.



Any other layer is fine.

Has anyone else experienced similar problems or an idea of how to debug this issue?

The picture is of PLA (BQ Easy Black) printed at 215 °C on the Ultrabase.
The the same issue with BQ Easy PLA red, turquoise and white (205 °C), Zaper PETG Brown 230 °C and HobbyKing translucent ABS Red (245 °C).

Changing the nozzle did not change anything.
Normally, such patterns are caused by too close printing to the bed (nozzle to bed distance too small), but considering the not fully flattened out extrusion lines (on glass I get a mirror shine fully closed surface) this does not seem to be the reason; you could try to increase the gap a little, in Ultimaker Cura this is very easily done with the Z Offset Setting plugin. Alternatively, you could use a little thicker paper to calibrate the bed levelling.