This problem happens when the current version of the file has one digit more than the previous version. E.g.:

src/java/nl/contrado/web/actions/OrderManagerAction.java - Rev. 1.100 (+19 - 17 lines)

The diff link is as follows:

http://<servername>/viewcvs/ordermanager/build/src/java/nl/contrado/web/actions/OrderManagerAction.java?r1=1.099&r2=1.100

When I click this link I get:

ERROR: rcsdiff did not return the correct version number in its output. (got "1.99" / "1.100", expected "1.099" / "1.100")

Aborting operation.

The problem is the "1.099" of the previous version. I think this should be 1.99, because that is what viewCVS uses as link:

http://<servername>/viewcvs/ordermanager/build/src/java/nl/contrado/web/actions/OrderManagerAction.java.diff?r1=1.99&r2=1.100

In case of a version 1.10 the previous version is 1.09. This causes the same problem as above.