Hi there,
is there a way to decrypt content.xml from an encrypted ODT file by using any other tool, like for example OpenSSL?
You’d need to evaluate the manifest to obtain the necessary parameters and key derivation algorithm, however those then could be fed to OpenSSL (or at all) no idea. There’s a quite good description of the general process available at Decrypting Open Document Format (ODF) Files accompanied by an (old but maybe still functional) Java library. For the ODF specification see v1.3 part 2 Packages, 3.4 Encryption and 4 Manifest File.
Update:
This OODecr script might give some inspiration how things maybe could be done with OpenSSL, but note that it’s about the old OpenOffice.org encryption and since then the ODF specification changed how things are done.