Never seen Silk erroneously put out that error, so the bitmap file is apparently locked by another application. Figure out who has the file(s) open if not yourself, close the file(s) and rerun Silk. Failing that, give the file a different name within your script.
I also began getting SilkTest file I/O errors when doing a large number of bitmap captures (1000+). In the end it turned out to be a hard drive issue: My c drive was running out of free space, and when I reclaimed some of that space the I/O errors disappeared completely.