The above is the start of the .bss section for uninitialized data in two .asm files. I then compile them as MS COFF into .obj files. After that I give them to polink together with kernel32.lib, which gives me the following errors:

Now firstly I didn't define any data sections at all as far as I know, and since the definitions of the bss sections are equal they should have the same flags, right?

What am I missing here? Is this something from kernel32.lib, is it bad, and (what) do I need to avoid it?

looks like one of the files is missing flag 0x80 (IMAGE_SCN_CNT_UNINITIALIZED_DATA), you might have put initialized data (like "dd 0" instead of "dd ?") in its .bss.

don't discribe bss section as data - it means initialized, just writable readable

Awwwww can you say blunder? Thank you, that's got to be it. I remember I switched the code generation (this is a compiler) to uninitialized data, but I forgot to change dd into rd.

I'll do that as well, is it how it initially was (before I encountered the problem), but I changed it to see if it had any effect.

I removed the data description, now polink says that section .bss is missing contents flag, assuming data.
Also, it turned out that I hadn't forgotten to change dd 0 into rd 1.
The link can't be completed because polink says "POLINK: fatal error: Internal error: bss sequence."