Bug in "Replace in File" in 5.6?
 

It seems that as of 5.6 the "Replace in File" action requires the target file to be writeable. The "ensure output file is writeable" has no effect.

Any ideas?

It is a bug introduced in 5.6. We plan to get a fix out next week. For now, you can work around it by adding the following line to the step's vbld_StepStarting script event:

vbld_MakeFileWriteable(Step.ExpProperty(Builder, "FileIn"))

Create the script event (if it doesn't exist) by opening the step properties dialog, clicking Script Editor, switching to the Step tab, clicking the Events button, and choosing vbld_StepStarting().