Thank you for the post.
FileMaker Pro 12.0v5 was never tested to be compatible with Mac OS X 10.10 Yosemite. For a full list of the operating system tested to be compatible, please review the following knowledge base articles:
"While these earlier versions may install and run on OS X Yosemite (10.10), you may encounter installation and stability issues for which there is no resolution."
Additionally, since you seem to indicate this happens when opening a database, check for third party plugins. Does this happen with a new database file (e.g. starter solution) with all third party plugins removed?
Review the following knowledge base article for the location of installed plugins:
No plug ins installed at all. Hangs also occur after the data base has been opened at pretty unpredictable times. This is annoying beyond belief and it's amazing that FM Pro 12 will not run on Yosemite. It isn't that old a version of FM to be obsoleted whether intentionally or not so soon.
Whether we like it or not, OS updates often leave versions of other software in the dust. I know this is "preaching to the choir" but it's always a good practice to check the compatibility of your important software before automatically installing an OS update. The flip side of this coin is better news. FMP 13 runs fine on my quad core Xeon with OS X 10.7.5. I expect that as newer versions of FMPA come out that this will no longer be the case.