there are two ways to solve the issue:
1- If you don't mind updating your FIJI then download and install the latest FIJI (bundled with java 8) from 'http://fiji.sc/'. Launch this new FIJI and run the updater to install the 'ScientiFig' update site (please see https://imagej.net/Following_an_update_site for a great tutorial on update sites)
2- If, on the other hand, you prefer to keep your 'aging' FIJI as it is (and/or prefer to keep using java 1.6), then you must enable the new 'Scientifig-deprecated' update site together with the 'Fiji-Legacy' update site while inactivating the 'ScientiFig' and the 'java-8' update sites.
Both solutions should work, but please let me know otherwise. Please also note that 'Scientifig-deprecated' is an old version of the software that contains a few known bugs and that will never be updated. I just created it for users that have an old FIJI bundled with java 6 and prefer not to install a more recent version (probably because they fear they may lose cool plugins during the transition, which is something I can totally understand).
For the record the issue comes from the fact that 'FIJI-legacy' contains batik 1.8 whereas 'java-8' contains batik 1.9 and some classes have been moved between the two versions of the batik library making it hard/impossible to use the same code with both versions (and thereby making the last version of SF not compatible with old FIJIs). Hopefully, I'll be able to find a better fix for that in the future rather than creating a deprecated update site, but that is the only solution that came to my mind for now.
Sorry for the inconvenience and thanks a lot for using this software.
ps: I have just modified the SF launcher so that it reports all problems it can detect and prints several guidelines on how to fix these issues