I can read your sentence in two ways without really trying too hard, the phrase..
temporarily suspended activity
..can be read both in a verby sense and a nouny sense. The meaning slightly changes when you read it as either one or the other. This isn't something others here have mentioned...
In the verby sense it says what you expect, that you have done something, and that thing you've done is temporarily suspended your active participation.
In the nouny sense, the meaning changes to something akin to "I own some activity that is suspended". This is, imo, why your reader is so convinced that this is what you meant.
The key problem here is that "activity" is both a thing that exists as data on the website, and can also mean your active participation on that website. This is what can confuse the matter. He didn't misread it, he didn't misinterpret, he did fail to realise your intended meaning though. The truth is your sentence has ambiguity because the word "activity" is ambiguous in its definition.
I suspect people here will use a dictionary to prove me wrong, but the fact is that
activity is a word commonly used in software circles and can mean more than one thing at once. For example;
"I went to the history page on the website to look at my activity info."
activity is both what you've done, and the data contained on the website, very ambiguous...
To answer your actual question, I would either avoid the word activity or give it a better qualification, for example;
"I've temporarily suspended my activity on SOru"
"I've temporarily suspended my active participation on SOru"