I thought you used to have to enter a 0 for the start value, but I thought they changed it so you could have a value of 1.
Sorry if it doesn't work, I was just working off memory.
Proud to be a Datanaut!
Try using a Start value of 0.1
I had struggled with a similar issue - I had set up a combo chart (line + stacked column) with a log scale and it worked fine... but when I made some unrelated changes to some of my measures, the scale changed back to linear!
When I checked the Format panel, the Y-axis option for Scale type no longer showed "Log". Instead, it had a yellow triangle, with the tooltip message: "Can't apply log scale - The axis changed to a linear to accommodate both negative and positive values. To use a log scale, remove one set".
NOTE: I am confident my measure values did NOT have any negative values - I used a table visual to manually inspect the data. Then I even wrapped my measure in an IF statement to check if the measure value is < 0 and, if so, set the measure value to 0. But I still got the same error message.
Based on the advice in this forum, I tried entering a non-zero value in the start value, but unfortunately that did not work.
However, if I changed the Start field to a value *between* 0 and 1 (I started with 0.1) then it worked! The Log scale was magically back.
I then discovered that by changing the Start value (0.01, 0.001, etc.) I could change how "steep" the log axis values changed.
I did an extensive web search about this issue, but this forum was the closest I found to a solution.
Hopefully this modified workaround works for others.
EDITED: to add my current version: 2.59.5135.421 64-bit (June 2018). I don't know if this workaround will work in other versions.