One of the techniques that I use when attacking a core world is to send a fleet that contains one or more transports to a cell that is between the core world and the shipyard associated with that core world (see the screen snapshot below).  Once I am positioned there, I select the transport(s) and attack the core world (assuming I have destroyed any defending ships prior to this point).  I then take the remaining ships in the fleet (which should already be automatically selected) and attack the shipyard.  After that, I immediately recombine the 2 back into a single fleet.

A bug seems to occur when I recombine the fleet BEFORE the Battle Report window showing the result of the combat with the shipyard appears.  When the bug occurs, the Battle Report window never appears and I cannot attack anything with any ship or fleet.  It is as though the battle with the shipyard never finished and the game will not allow any other combat to start until the shipyard battle is finished, which it never will.

I have saved a game that you can use to recreate this bug.  It is saved here:

https://www.dropbox.com/s/3rb1pw8tzijl0da/Battle%20Bug%20Under%201.11.366105.GC4Sav?dl=0

To recreate the bug, load the saved game then:

1. Move so that you can see Sheratan IV as shown in the screen snapshot below (it is in the left side of the area controlled by the Arceans at the top of the universe)
2. Select my fleet of 17 ships that is positioned between Sheratan IV and the shipyard
3. Select the 2 Transports from within the fleet
4. With the 2 Transports selected, right-click on Sheratan IV which should start the invasion
   1. This will split the fleet of 17 ships into 2 fleets, one containing the 2 transports and 1 containing the other 15 ships, both occupying the same cell.
5. At this point the remaining 15 ships from the fleet should be selected.  Right-click on the shipyard and they should attack the shipyard.
6. IMMEDIATELY click on the fleet management icon, then quickly select both of the fleets, click on the icon the combine them, and click the Done button.
7. If you were able to complete step 6 BEFORE the Battle Report window appears, you will no longer be able to attack anything else with any ship or fleet.  You will know if you were quick enough if the Battle Report window never appears.
   1. There is a fleet of 5 ships slightly above Sheratan II that you can use to try and attack the starbase right next to it to show that attacks are no longer possible.

If you were unable to duplicate the bug, reload the save and try again.  I have been able to consistently replicate the issue from the save file as long as I completed the fleet recombination step before the battle results screen appeared.

I recognize that this is an obscure bug, but it is something that the devs should look at since it is most likely a thread synchronization issue that could result in other undesired side-effects besides this one.