Why does regex with \{ work with egrep, but not with sed?

Given a text like this

./RFF_09 -f${FILE} -c${COND}

inside a file, this egrep command will correctly match:

egrep './RFF(.*) (.*)-c\\$\{COND\}' file

but this sed command will not

sed -n "s:'./RFF(.*) (.*)-c\\$\{COND\}':./RFF$1 $2-cRFF$1:gp"

It will fail with sed: -e expression #1, char 38: invalid content of \{\}. I've also tried with

sed -n "s:'./RFF(.*) (.*)-c\\$\{COND\}':DUMMY:gp" file
sed -n s:'./RFF(.*) (.*)-c\\$\{COND\}':DUMMY:gp file

with the same result.

sed -n "s:'./RFF(.*) (.*)-c\\$\\{COND\\}':DUMMY:gp" file

Will not give me an error message, but will not match.

What am I doing wrong? Or better: How can I replace the text as suggested by the original command? I'm using rather old versions of sed (4.1.2) and egrep (2.5.1), so a workaround is appreciated even if you can't reproduce the error with newer versions.