EDIT #2: Turns out restarting Arma fixes the bug. Therefore, this bug is far less serious than I thought.

>Link to .rpt
.rpt file - dated 2020-01-01[www.dropbox.com]

>Environment
Singleplayer

>Mods
CBA_A3
RHS USAF
RHS AFRF
RHS GREF
ACE - Antistasi Edition
Advanced Rappelling
Advanced Urban Rappelling
Advanced Sling Loading
Advanced Towing
Athena - Arma 2nd Screen

>Modified mission file
Mission file is unchanged.

>DESCRIPTION
I've run into an irritating bug that results in a vehicle placement dialog that refuses to go away.
I discovered just now that the Purchase Boat option at the Kavala Sea Port flagpole is unavailable, meaning that's probably the culprit. I don't know how exactly to replicate it, but going there first should shed some light on this issue.
Some screenshots:
Screenshot 1: Kavala Sea Port[i.imgur.com]
Screenshot 2: Neri Outpost[i.imgur.com]

Solutions I have tried:
Pressing Enter
Holding Enter
Suicide
Placing vehicles from the faction garage, purchasing vehicles, going back to Kavala Seaport to try to purchase the boat for real
Restarting the scenario, loading a previous save
Restarting the scenario, not loading a previous save
Saving the scenario while the dialog is not visible, then restarting the game
Restarting Arma is the only way I have found that works, but it's an easy solution

Of specific interest in the .rpt file was the following:
6:53:51 Error position: <garageVeh};
garageVeh = nil;
comprado = >
6:53:51 Error Undefined variable in expression: garageveh
6:53:51 File REINF\addFIAveh.sqf [A3A_fnc_addFIAveh]..., line 117
6:53:51 Error in expression <nil;garageVeh = nil};
waitUntil {isNull garageVeh};
garageVeh = nil;
comprado = >