Hello
As the file is too big, it's easier to past here an example of the ohm question.
This file is from a 3.3.4 moodle backup, mbz, without any special process. I attempt to restore it into a 3.5.2 moodle installation.
I achieved to restore the full course after deleting 20 questions like this one, with the same error message, for others numerical units (like cm2, volts, MJ,...)
<questions>
<question id="41387">
<parent>0</parent>
<name>Q1</name>
<questiontext><p>Calculer la résistance du conducteur ohmique sachant que la tension à ses bornes vaut U = {U} V et l'intensité I = {I} mA</p></questiontext>
<questiontextformat>1</questiontextformat>
<generalfeedback><p>R = {U}/({I}x10<sup>-3</sup>)</p></generalfeedback>
<generalfeedbackformat>1</generalfeedbackformat>
<defaultmark>1.0000000</defaultmark>
<penalty>.3333333</penalty>
<qtype>calculatedsimple</qtype>
<length>1</length>
<stamp>0680034t.moodle.entea.fr+180221155456+mpkovI</stamp>
<version>0680034t.moodle.entea.fr+180221155457+19xGWK</version>
<hidden>0</hidden>
<timecreated>1519228496</timecreated>
<timemodified>1519228496</timemodified>
<createdby>955</createdby>
<modifiedby>955</modifiedby>
<plugin_qtype_calculatedsimple_question>
<answers>
<answer id="51393">
<answertext>{U}/{I}*1000</answertext>
<answerformat>0</answerformat>
<fraction>1.0000000</fraction>
<feedback></feedback>
<feedbackformat>1</feedbackformat>
</answer>
</answers>
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<multiplier>1.00000000000000000000</multiplier>
<unit>Ohms</unit>
</numerical_unit>
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<multiplier>1.00000000000000000000</multiplier>
<unit>Ohm</unit>
</numerical_unit>
<numerical_unit id="528">
<multiplier>1.00000000000000000000</multiplier>
<unit>ohm</unit>
</numerical_unit>
<numerical_unit id="529">
<multiplier>1.00000000000000000000</multiplier>
<unit>ohms</unit>
</numerical_unit>
<numerical_unit id="530">
<multiplier>.00100000000000000000</multiplier>
<unit>kO</unit>
</numerical_unit>
</numerical_units>
<numerical_options>
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<showunits>0</showunits>
<unitsleft>0</unitsleft>
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<unitpenalty>1.0000000</unitpenalty>
</numerical_option>
</numerical_options>
<dataset_definitions>
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<category>0</category>
<name>U</name>
<type>1</type>
<options>uniform:1.0:10.0:1</options>
<itemcount>30</itemcount>
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<tolerance>0.5</tolerance>
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<correctanswerlength>2</correctanswerlength>
<correctanswerformat>2</correctanswerformat>
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<question_hints>
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<tags>
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