Problem

-- trying to add a new component of type Document

-- "Loading, please wait..." is displayed in the UI

-- CN is showing:

2018-08-08 18:38:19,600 - ERROR [qtp462167917-573] (IntegratorHostController) - Could not retrieve Components from Integrator
b2bx.server.B2BXException: java.net.SocketTimeoutException: Read timed out

In Integrator trace:

ERROR :20180808:18.38.12.99:r4edi(b2bi_clusternode):B2Bi Cluster Node:239:Node 'xxxxxxxx' is not available. Timer started 60 seconds ago.

Resolution

-- the root cause must be investigated (slowness due to the OS, high number of MBCs etc...)

-- but as a workaround you can add b2bxserver.socket.timeout timer on Integrator engine node from the System management.
-- The default value for b2bxserver.socket.timeout is 60 seconds. Give a try with 120 and test again