I had a Bitnami install for a few months, then moved to simple Ubuntu.
Bitnami is nothing special, unless your errors are with fundamental installation issues, like getting PHP or MySQL to run, then your problem is probably not specific to Bitnami.
There are many, many posts out there explaining how to fix permissions on your install - I would start with that. When random bits of the application that work for everybody stop working for you, 99% of the times the problem is permissions degradation.
Try going to admin -> upgrade wizard and run the first step only, which is permissions check. Do you get files not writeable there?