A frequent error  with the skining,  properly the next one: "The+Controls+collection+cannot+be+modified+because+the+control+contains+code+blocks+(i.e.+%3c%25+...+%25%3e).&content=0"
it's caused for content panes ID. if you are using in your ascx file a design with more than the 3 basic panes (left, content, rigth) you most start with the use of these ones, specially be carefull of stablish the "content pane" without this one the installing of your skin could even collapse your DNN installation, that pane must be contained in your design (belive me I´m a designer not a developer). For the avoiding ot this, use the preview option before the installing.

Finally good luck I hope this were your problem.