What ester exactly? Some esters are especially suited for acidolysis (e.g., tert butyl and trityl groups).

In the mean time, a similar question was asked on research gate and you can check the answers there:

https://www.researchgate.net/post/What_is_a_simple_way_to_convert_an_ester_into_carboxylic_acid

https://www.researchgate.net/post/What_is_a_simple_way_to_convert_an_ester_into_carboxylic_acid

What is the problem? Is the ester not reacting or is the isoxazole reacting as well?

it depends upon the group attached in acid part  

depending on that there will be acidic or basic hydrolysis

You can try TMSOK or Tin hydroxide

Reaction conditions depend on presence of base/acid sensitive groups.

Without them we used a following procedure: 1. treat an ester with ca. 5% ethanolic-aqueous NaOH (4 equivalents) and heat the mixture at reflux with a TLC control. It’s not necessary to dissolve the ester in the beginning, dissolving occurs during the reaction, sometimes addition of THF is helpful; 2. Dissolve the obtained salt in water (if it separates), extract 3 times with hexanes or ethyl ether to get rid of small amount of impurities, acidify with dilute HCl (with NH4Cl for very acid sensitive compounds) and extract with dichloromethane to get an acid sufficiently pure for most purposes.

Good luck.

Thank you for your suggestion, I have tried the same but not able to isolate the pure acid.