Ody Galmap has a number of issues related to plotting and filters. I can't positively tie the OP's issue ao any that I know about, however one thing that has n ot explicitly been ruled out above could be it I suspect it would have been tried, just not reported above.
Select Realistic map mode, then in route settings toggle to Economical, then back to Fastest route, then plot somewhere else that WILL plot, then try re-plotting to the unplottable destination. Has that been tried?
Additionally (I know this isn't the point of this thread) If the system was targeted (rather than plotted to) could it be jumped to?
The issues I'm aware of:
Upon login (from launcher certainly, not sure if on relog via Main menu), Neither "Fastest route" nor "Economical route" are visibly checked under route settings. Galmap seems to behave something but not exactly like fastest route. Issues being - no matter what was selected before, it always acts like Jet cone boot is allowed, secondly it can plot some jump segments over-range for the ship in question (not taking into account Laden mass maybe??? My Patting Krait Phantom has 69.5Ly range fully laden (incl. limpets) - I've seen this bug plot jump segments of over 72Ly, or route me through White dwarfs (with next jump requiring jet cone bost) where my filters and jet cone boost setting should denny that.
Once the player has positively selected Fastest route and their choice of "Use jet-cone boost", those settings get honoured until next re-log (I think via desktop but not 100% sure here). However when revisiting the route settings form, Fastest/economical will both be deselected. Selecing fastest again will populate the "Use jet cone boost" setting with the players last choice (showing THAT was remembered)
Once bitten by the above bug (and fastest manually selected), the "Recalculate" button might or might not work. Often I find that I need to trash the currently selected route (plot somewhere else) and then replot to my destination. Additionally just clearing the route and replotting again is not enough - sometimes I need to plot somewhere else, then replot to my destination. Sometimes if I've erroneously been sent to a WD system with the next jump requiriong boost, replotting in any way might not clear the need for the boost, I have to manually target an adjacent system to restore normal function.
Unrelated to this plotting issue but the other big gripe with Galaxy map is that (again on log in) and additionally if some filter options are moused over or explored without obviously changing anyhing) fiters can get lost or corrupted. i see this a lot, bit myself, and from the number of Ody fuel calls I attend where the stricken commander knows KGBFOAM and the star class filter, but simply believes they forgot to set it. A particuarly insidious incarnation of this is that with the star class filter, the "Apply filter to route" option is often the setting that goes AWOL, and that setting now needs to be scrolled down to. You can glance at the star class filter, think "Top 7 stars selected ....yep... Filter still there" and not realse that the filter is not being applied to route any more.
Of course as these bugs are not related to shooting things, the issues on the tracker don't reach critical mass. Even non shooty bugs that do reach tier 4/5 voting (the rats have had navlock issues for the best part of a year now) are not fixed (well FDev did have a go at fixing that issue, and made the problem WORSE instead of better...) Now there is a follow up Tier 5 issue raised, but a few patches have gone past with no further resolution. If I get the opportunity, I will raise this issue with the devs at Lavecon.
To wrap up (if you are still awake), The reason for my suggestion above was to ensure that the route setting and filtering was reset to a known state, then to try to disassociate the route that had previously tried from any internal caching (the plot somewhere else), then to replot cleanly giving ourselves the best chance of using the corrected settings.
