The red X items indicate that CC3+ can't find the resources on the path stored in the file. Most likely, the image files in question aren't included with CC3+ and were specified as "relative to CC3" in the original drawing (using LIST on one of the problem files will likely show its file name starting with the # character).
The red crosses are Symbol Set 1 symbols used int he map. Symbol Set 1 is not compatible with CC3+ yet.
That said, you can make the map viewable in CC3+ by copying the bitmap resources from the CC3 program folder (usually c:\Program Files (x86)\Profantasy\CC3) over to the CC3+ programdata folder (usually c:\ProgramData\Profantasy\CC3Plus). For SS1 one these would be in /Symbols/Maps/SS1/ and /Bitmaps/Tiles/Overland/Overland SS 1a/
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That said, you can make the map viewable in CC3+ by copying the bitmap resources from the CC3 program folder (usually c:\Program Files (x86)\Profantasy\CC3) over to the CC3+ programdata folder (usually c:\ProgramData\Profantasy\CC3Plus). For SS1 one these would be in
/Symbols/Maps/SS1/ and /Bitmaps/Tiles/Overland/Overland SS 1a/