If Strict Trade is enabled, Pioneers presently won't let you trade if you've played a Road Building card earlier in your turn. This is incorrect - see the official website FAQ. Only normal building (spending your resource cards to build) counts as the build phase and thus precludes further trading.
Thanks for reporting the issue.
I've implemented a fix in svn commit r2205, which will be part of the upcoming release of Pioneers 15.5
Is released in Pioneers 15.5