Can't include theorem in my poster

If I remove the loading of amsthm there is no error.

If you need to style your theorems, you can use ntheorem.

Saving a piece of advice for the future generations: I had a similar issue when writing a document with lots of custom theorem environments (all created with amsthm). When I first used one of them, this issue appeared. The code that defined it was

\newtheorem{escoliox}[teoremax]{Escólio}   
\newenvironment{escolio}
    {\pushQED{\qed}\renewcommand{\qedsymbol}{$\square$}\escoliox}
    {\popQED\escoliox}

After inspecting the code for some time and testing with other custom environments, I discovered the problem was that i hadn't closed the escoliox environment when defing escolio. Therefore, when changing the first code to

\newtheorem{escoliox}[teoremax]{Escólio}   
\newenvironment{escolio}
    {\pushQED{\qed}\renewcommand{\qedsymbol}{$\square$}\escoliox}
    {\popQED\endescoliox}

the problem dissapeared. Oh, I was using Overleaf v2 (which is Overleaf + ShareLaTeX), but I believe this things might not be relevant for this issue.