GPO 3 and 4, and JABB 2 and 3 should still work fine with Sibelius 8.5.  Simply install the soundsets if they're not already there.

http://www.sibelius.com/helpcenter/resources/soundsets.html

https://makemusic.zendesk.com/hc/en-us/articles/218211147-Sibelius-sound-sets-for-Garritan-libraries

So far, every version of Sibelius I've installed from 6 forward already included most of the soundsets, including the older JABB 2.0 and 2.0 Lite variants.

Of course you'll need to set everything up properly in your VST Setup to make use of them.

GPO5 does not have an 'official' soundset for Sibelius.  I've made my own, and you can get it here:

https://makemusic.zendesk.com/hc/en-us/community/posts/115000169548-GPO5-Sibelius-Soundset-Version-22-

My personal soundset isn't perfect by any means, but it does at least make GPO 5 functional in Sibelius, and it's a solid start towards tweaking and making it your own.

Thanks for the quick response.

I'll try following your instructions.

Viktor,

I forgot to ask if you're on Mac or PC?

I don't have Mac experience, but I've read that sometimes people have issues when upgrading/updating various Mac OSes (It can happen for Windows too).  Some of the newer one might not be compatible with older installers/versions.  With that in mind, if you still have trouble with ARIA based instruments after insuring the 'soundsets' are installed for Sibelius, log into your MakeMusic Account, and in the support area search for the latest updates for the ARIA Player.  Once you've established you're up to date there, open ARIA in stand alone mode, and in the info tab check for ARIA Engine Updates.

It's Mac.

Anyway, thanks for help, it was very useful, so it seems that the problem is solved for now!

V.