Thank you, I have solved the problem, which has to do with a plug-in called “Custom Permalinks”
1. Problem is caused when slug of page in question is changed using plugin “custom permalinks”
2. Solving problem is done by:
a. deactivating plugin
b. seeing what the “default” slug is for page in question
c. copying “default” slug
d. reactivating “Custom Permalinks”
e. using “default” slug exactly as it appears without the custom permalinks plugin
f. save
g. change frontpage to this page.