With dune 1.8.2.
Repro:

• an empty hello_world.ml file
• an empty C file with a '.' dot in the name, like file.a.c
• a dune file:

(library
 (name hello_world)
 (c_names file.a)
)

Then with dune build one gets the misleading error:

File "dune", line 3, characters 10-16:
3 |  (c_names file.a)
              ^^^^^^
Error: file.a does not exist as a C source. file.a.c must be present

Changing the name to file_a.c for example works.