Check that both Tortoise and SVN are the same client versions. In explorer do Right Click -> Tortoise SVN -> About. Your version info will look like this:
TortoiseSVN 1.6.5, Build 16974 - 32 Bit , 2009/08/20 08:13:46
Subversion 1.6.5,
apr 1.3.8
apr-utils 1.3.9
neon 0.28.6
OpenSSL 0.9.8k 25 Mar 2009
zlib 1.2.3
So mine is client version "Subversion 1.6.5". From the command line do:
svn --version
You'll get something like this:
svn, version 1.6.4 (r38063)
compiled Aug 5 2009, 12:42:52
Copyright (C) 2000-2009 CollabNet.
Small version differences are usually OK. Big differences are likely to cause you a problem.
The other thing to check is that you are storing SVN information in ".svn" directories for both versions. Tortoise lets you change to use "_svn" instead of ".svn" (in Tortoise SVN -> Settings -> General). This may cause you a problem.