Touching the file did not work for me.

I ran gpgv under strace, though, and saw this:

access("/var/cache/debmirror/.gnupg/trustedkeys.kbx", W_OK) = 0 write(2, "gpgv: can't allocate lock for '/"..., 74gpgv: can't allocate lock for '/var/cache/debmirror/.gnupg/trustedkeys.kbx) = 74 write(2, "'\n", 2' ) = 2

From this, I inferred that it was complaining not about what it could not do, but about what it could.

Removing write permissions from the file eliminated the (misleading) "error" message ... which also did not prevent the program from continuing.

Touching the file did not work for me.

I ran gpgv under strace, though, and saw this:

11857 close(3)                          = 0                          
11857 access("/var/cache/debmirror/.gnupg/trustedkeys.kbx", F_OK) = 0  
11857 access("/var/cache/debmirror/.gnupg/trustedkeys.kbx", R_OK) = 0  
11857 access("/var/cache/debmirror/.gnupg/trustedkeys.kbx", W_OK) = 0  
11857 write(2, "gpgv: can't allocate lock for '/"..., 74) = 74  
11857 write(2, "'\n", 2)                = 2  
11857 openat(AT_FDCWD, "Release.gpg", O_RDONLY) = 3  
11857 read(3, "-----BEGIN PGP SIGNATURE-----\n\ni"..., 8192) = 1760  
11857 read(3, "", 8192)                 = 0  
11857 openat(AT_FDCWD, "Release", O_RDONLY) = 4  
11857 read(4, "Origin: Debian\nLabel: Debian\nSui"..., 8192) = 8192  
11857 read(4, "\n 345032ae178d52433929ef4b76962a"..., 8192) = 8192

From this, I inferred that it was complaining not about what it could not do, but about what it could.

Removing write permissions from the file eliminated the (misleading) "error" message ... which also did not prevent the program from continuing.