According to the 6th edition of the APA Publication Manual (p. 84), the use of since is more precise when it is used to refer only to time (to mean “after”). You should replace it with because when that is what is really meant. Examples of both terms being used correctly are listed below:
Since Smith’s (2000) research was conducted, many additional researchers have achieved similar results.
The participants were excluded from the experiment because they did not meet the inclusion criteria.
Because the data were not complete, the results were excluded from the study.
No additional testing has been performed since the last experiment.
This is a strict interpretation and application in business writing should have some contextual freedom. Grammar Girl wisely argues that sometimes "since" and "because" can be synonyms:
Strict grammarians may not like it, but “since” and "because" can be synonyms. “Since I love you, let’s get married” means the same thing as “Because I love you, let’s get married.” (Yes, you can use “because” at the beginning of a sentence.)
Fussy grammarians might be a teensy bit right in some cases, though. The word “since” often refers to how much time has passed, as in “Since yesterday, all I’ve thought about is you.” Sometimes, a sentence with “since” can be interpreted in two ways, and that is when you should avoid using “since” to mean “because.” Take this ambiguous sentence:
“Since they spoke, she’s had second thoughts.” (“Since” could mean “from the time that they spoke” or “because they spoke.”)
My recommendation is simply to be aware of the distinction between "since" and "because" and use care to use "since" when it specifically connotes the passing of time.