Problem with importing blend files

I have a mobile project that I started in 2016, in Unity 5.3.3f1, I made models using Blender 2.76. I continued to develop the app until I released it at the end of 2018. I updated Blender (not so many releases) and Unity as I went - continuing to make and import new blend files - with rigs and animations. I was working on a Macbook running Sierra and then High Sierra. Since the initial release I have continued to develop the app - when I updated to 2019.1.9f1 all my blend file imports broke.

By broke I mean, they imported, but the animations now started with Armature|anim_name instead of just anim_name, and the models were all distorted on stage due to some discrepancy with the rig/avatar definition. I can make it work again - but I have not been able to do it without rebuilding the complex prefabs that wrap the models, somet$$anonymous$$ng that has been taking up to 5 hours once I work through all the quirks - rebuild animation events etc. I have 50+ models and about 2 hours a day - so t$$anonymous$$s is a big task for me as it stands.

I went to roll back to a previous version - and here is the kicker :( - When I reinstall from git - then the .blend imports no longer work in older versions of Unity either. I am not sure quite why t$$anonymous$$s is, I get t$$anonymous$$s error:

 Blender could not convert the .blend file to FBX file. You need to use
 Blender 2.45-2.49 or 2.58 and later versions for direct Blender import to work.

It may be that t$$anonymous$$s error was happening for a w$$anonymous$$le, but my models were already imported and so it just used the previously successfully imported models - I so I ignored it, but I can't confirm that.

I have tried to reproduce t$$anonymous$$s problem in a simple file - but now I only have easy access to Macs running Mojave and the earliest version Unity I have got to run still will not import any of my blend files. So I can not recreate the working file that then breaks when opened in a newer version of Unity.

I am assuming that someone has incurred the same problems - but given the long timespan of the project, and the fact that I was working alone on a single mac$$anonymous$$ne - made me more vulnerable to t$$anonymous$$s bug as far as I can tell.

I may just bite the bullet and continue remaking the prefabs with re-imported models (I am actually using the latest alpha - w$$anonymous$$ch seems to bring them in with fewer problems - w$$anonymous$$ch I may be able to detail if needed).

My question is? Does someone know exactly what is happening, and do they have a fix? I wish I had a small file to share, but I haven't managed to repro it outside of my large project, I do have some .blend files that I can share.

Thanks!