<location id="twisty_river" seed="12333647" environment="custom" map_location="0.655,0.52"/>
c:\Users\XXX\Documents\DawnOfMan\Scenarios\Custom\custom.scn.xml
c:\Users\XXX\Documents\DawnOfMan\Scenarios\Custom\Environments\custom.xml

error message: ERROR: SerializationException: Error at path: Environments/custom
Could not find file or resource

Seems like the content or more likely the name of your custom.xml is not right. Is it named custom.xml or Custom.xml ?
you can copy the full error text. If i leave the file without content it gives me this:

ERROR: SerializationException: Error at path: C:\Users\user\Documents/DawnOfMan/Scenarios\custom/Environments/custom.xml
Document element did not appear. Line 1, position 1.

please paste this in and test it:

<?xml version="1.0" encoding="utf-8"?>
<environment>
<noise_amplitudes values="0"/>
<trees values="Oak"/>
<trees_everywhere value="true"/>
<seasons>
<season id="Spring" setup_id="Spring" duration="0.25" precipitation_chance="0.0" windy_chance="0.0" very_windy_chance="0.0">
<min_temperature value="5"/>
<max_temperature value="25"/>
</season>

<season id="Summer" setup_id="Summer" duration="0.25" precipitation_chance="0.0" windy_chance="0.0" very_windy_chance="0.0">
<min_temperature value="20"/>
<max_temperature value="35"/>
<min_wind value="0"/>
<max_wind value="5"/>
</season>

<season id="Fall" setup_id="Fall" duration="0.25" precipitation_chance="0.0" windy_chance="0.0" very_windy_chance="0.0">
<min_temperature value="5"/>
<max_temperature value="25"/>
</season>

<season id="Winter" setup_id="Winter" snow_setup_id="WinterSnow" duration="0.25" precipitation_chance="0.0" windy_chance="0.0" very_windy_chance="0.0">
<min_temperature value="-15"/>
<max_temperature value="10.0"/>
</season>
</seasons>
</environment>

THX, now ok, dont know what was wrong, thx again

probably an error in the values of the environment custom.xml Good Luck!