I would double check the "pubmed-integration.xml" file that is *installed* in the [dspace]/config/spring/api/ folder (note: this is not the "dspace-src" directory). According to that error message, DSpace cannot find that "maxRetry" setting. So, it's likely that either your "pubmed-integration.xml" file is missing that setting, or perhaps the file has an invalid character (which makes it not be valid XML), which could cause DSpace not to be able to parse that configuration file properly.
It's also possible that your upgrade to 6.3 did not properly replace all the Spring configuration in your [dspace]/config/spring/api/ folder (it should have done this during the "ant update" step of your upgrade). If that's the case, assuming you haven't modified the Spring configuration manually, it is safe to *delete* the entire [dspace]/config/spring/api/ folder, and rerun "ant update" (which will replace it with the 6.3 version of the configurations).
In any case, this error definitely sounds like an issue in your configuration file. It would not be caused by a firewall or proxy problem.
Tim