Possible causes
If sudden foaming occurs while nothing has changed with the design of the lube system or reservoir, it is most likely caused by contamination. Sources of contamination could be water or solids (dirt, lacquer or deposits).
Other possible causes of the foaming include the lubricant being mixed with incompatible lubricants or greases, mechanical issues that cause excessive aeration of the lubricant, or an excessively fine filtration system that separates the defoamant additive from the lubricant.
Possible solutions
Firstly, identify the root cause of the problem. Usually an oil change will be required, and in the case of severe contamination complete flushing of the system might be needed as well.
In a system where foam is generated mechanically, switching to synthetic oil may help as they are less likely to foam than mineral oil based lubricants. After switching from mineral oil to synthetic oil the synthetic oil will act as a detergent, therefore when cleaning the system of deposits it may start to foam again. If this happens, an extra oil change is required to ensure a fresh and cleaned system.
If contamination with another lubricant is the issue, additional training or the implementation of a proper lubrication program with, for example, colour coding is recommended.