In order to reproduce:

let g:OmniSharp_start_server = 0
let g:OmniSharp_selector_ui = ''

Have 2 solutions Foo.sln and Bar.sln for a given Foo.cs file in some csproj

Analysis:
At the time of writing, the ftplugin file has the functions OmniSharp#actions#highlight#Buffer and OmniSharp#actions#buffer#Update run whenever cursor goes into/leaves from a *.cs buffer, and also on some other events.

On each of the events, these functions all call OmniSharp#GetHost(...) (from OmniSharp#stdio#Request).

OmniSharp#GetHost(...) then calls OmniSharp#FindSolutionOrDir which ends up prompting the following message:

Solution:
1. some\path\to\Foo.sln
2. some\path\to\Bar.sln
Type number and <Enter> or click with the mouse (q or empty cancels):

hence the prompt being spammed and blocking the UI

Additional information:

• I have successfully got rid of the issue by adding if OmniSharp#IsAnyServerRunning() before calling OmniSharp#actions#highlight#Buffer and OmniSharp#actions#buffer#Update in the ftplugin file, but wasn't too sure if this was the way to go for fixing this behaviour
• the issue also happens from sharpenup when the function sharpenup#codeactions#Count() calls OmniSharp#actions#codeactions#Count(opts) ; adding if OmniSharp#IsAnyServerRunning() before the call does the trick.
• adding if !OmniSharp#IsAnyServerRunning() | return | endif inside OmniSharp#stdio#Request(command, opts) is another solution that works, but probably has implications on the performance side of things.