MacBook – Connecting new 2014 Mac Mini and 2009 Mac Book Pro thru one VGA display

The error sounds like it's unable to resolve the alias to the original source. What I would do is remove the shared folder(s) from the Preferences on the Mac Mini, and recreate a preference for a specific folder (ie. Public), with further testing. If the issue were to persist, I'd create another User and test from there.

The mini DisplayPort to DVI adapter will work great, so your 27" monitor is in.

The VGA monitor is problematic. HDMI and DVI share almost identical digital video signals, so any HDMI to DVI adapter (like the included one) has few (if any) conversion chips in it and the output DVI port will be DVI-D (digital only). DVI to VGA adapters also don't have conversion chips because they use DVI-I or DVI-A ports (A is analog only and I id both analog and digital): they take the DVI analog signal and move it straight to the VGA port. Unfortunately, that means that you cannot go from HDMI to VGA via DVI. You'll need a more expensive converter like this or this.

tl;dr: You can't go from HDMI to VGA without a converter; see the links for examples. For the dual-link DVI you need, the adapter you mentioned that will use the Thunderbolt port will work excellently.