From grepping in the build results it looks like it's a

sky130_fd_sc_hd__buf_1

but at what point are these numeric identifiers created, and from what?

Somebody needs to add

autoname

to the openlane Yosys script...

The numbers at the end are drive strength or number of inputs

I interpreted this as meaning things like

_20535_

and

_20164_

from Olof's previous message

Those look like wire names

Created by Yosys.

Autoname sounds useful

The names it generates are..less than useful, but, still a bit better than the numeric wire names Yosys normally creates

I traced them to be the clock signals. Is that yosys adding clock buffers or the CTS stage? From the clock input net to the point of failure I counted one sky130_fd_sc_hd__clkbuf_16 followed by three sky130_fd_sc_hd__clkbuf_2 in a row, three additional sky130_fd_sc_hd__clkbuf_16, one sky130_fd_sc_hd__inv_2 and finally one sky130_fd_sc_hd__buf_1

CTS inserts clock buffers

Then I guess yosys autoname wouldn't help here?