Documentation 
Section: 7.2.4.4.2. Continuous Axes: Linear and Radial
None of this info is written in the manual, albeit it should.

Correct version:
the "Minor ticks" property of an axis has a meaning that depends on the type of scale:
- for linear scale, the property indicates the distance between minor ticks
- for log scale, the property indicates the number of minor ticks in a decade

The "Major ticks" means:
- for linear scale, the distance between major ticks
- for log scale, the number of decades between two major ticks

for both minor and major ticks in log scale, the number should be a positive integer.

Other information:
This bug is reported in Ubuntu Launchpad as 
https://bugs.launchpad.net/ubuntu/+source/gnumeric/+bug/246177