Tex capacity exceeded, sorry [input stack size = 5000]

It seems that the PhDbiblio-url2 bibliography style does not want the url field to contain \url{...}. It takes care of the url formating itself, enclosing the contents of the url field in \href{...}.

If you have something like

url = {\url{http://example.com/}};

in you .bib file, the PhDbiblio-url2 style translates that into

\href{\url{http://example.com/}}

which creates an infinite loop in the hyperref package.

The proper way to include url in the bibliography, when using this style, is just

url = {http://example.com};

Not a useful general answer, but a specific case in Beamer:

A \begin{block} statement (without a name option) will cause this error. The correct format is \begin{block}{Block title}, or \begin{block}{} if you want no title.

There's a config file where some numbers about "memory" TeX uses are written, I have it into /etc/texmf/texmf.cnf; you may need to change some value and regenerate formats.

stack_size = 5000        % simultaneous input sources

Add

Of course the problem may arise from a bug in TeX macros you're using, that consumes all the stack; so we need more information.

EDIT

If you want to edit that file, avoid editing /etc/texmf/texmf.cnf as always in configuration files. Instead edit the specific file in /etc/texmf/texmf.d/ - on Ubuntu currently /etc/texmf/texmf.d/95NonPath.cnf.

After editing the file you have to execute update-texmf to rewrite the configuration file.

EDIT2

Often it happens you can't have access to /etc or other system directories, so you need your own cnf file (and TeX tree). This link talks about it.